Linaro-mm-sig

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[PATCH] dma-buf: fix timeout handling in dma_resv_wait_timeout v2

by Christian König

Even the kerneldoc says that with a zero timeout the function should not wait for anything, but still return 1 to indicate that the fences are signaled now. Unfortunately that isn't what was implemented, instead of only returning 1 we also waited for at least one jiffies. Fix that by adjusting the handling to what the function is actually documented to do. v2: improve code readability Reported-by: Marek Olšák <marek.olsak(a)amd.com> Reported-by: Lucas Stach <l.stach(a)pengutronix.de> Signed-off-by: Christian König <christian.koenig(a)amd.com> Cc: <stable(a)vger.kernel.org> --- drivers/dma-buf/dma-resv.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index 5f8d010516f0..c78cdae3deaf 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -684,11 +684,13 @@ long dma_resv_wait_timeout(struct dma_resv *obj, enum dma_resv_usage usage, dma_resv_iter_begin(&cursor, obj, usage); dma_resv_for_each_fence_unlocked(&cursor, fence) { - ret = dma_fence_wait_timeout(fence, intr, ret); - if (ret <= 0) { - dma_resv_iter_end(&cursor); - return ret; - } + ret = dma_fence_wait_timeout(fence, intr, timeout); + if (ret <= 0) + break; + + /* Even for zero timeout the return value is 1 */ + if (timeout) + timeout = ret; } dma_resv_iter_end(&cursor); -- 2.34.1

4 months, 4 weeks

[PATCH] dma-buf: fix timeout handling in dma_resv_wait_timeout

by Christian König

Even the kerneldoc says that with a zero timeout the function should not wait for anything, but still return 1 to indicate that the fences are signaled now. Unfortunately that isn't what was implemented, instead of only returning 1 we also waited for at least one jiffies. Fix that by adjusting the handling to what the function is actually documented to do. Reported-by: Marek Olšák <marek.olsak(a)amd.com> Reported-by: Lucas Stach <l.stach(a)pengutronix.de> Signed-off-by: Christian König <christian.koenig(a)amd.com> Cc: <stable(a)vger.kernel.org> --- drivers/dma-buf/dma-resv.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index 5f8d010516f0..ae92d9d2394d 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -684,11 +684,12 @@ long dma_resv_wait_timeout(struct dma_resv *obj, enum dma_resv_usage usage, dma_resv_iter_begin(&cursor, obj, usage); dma_resv_for_each_fence_unlocked(&cursor, fence) { - ret = dma_fence_wait_timeout(fence, intr, ret); - if (ret <= 0) { - dma_resv_iter_end(&cursor); - return ret; - } + ret = dma_fence_wait_timeout(fence, intr, timeout); + if (ret <= 0) + break; + + /* Even for zero timeout the return value is 1 */ + timeout = min(timeout, ret); } dma_resv_iter_end(&cursor); -- 2.34.1

4 months, 4 weeks

Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

by Christian König

Am 22.01.25 um 12:04 schrieb Simona Vetter: > On Tue, Jan 21, 2025 at 01:36:33PM -0400, Jason Gunthorpe wrote: >> On Tue, Jan 21, 2025 at 05:11:32PM +0100, Simona Vetter wrote: >>> On Mon, Jan 20, 2025 at 03:48:04PM -0400, Jason Gunthorpe wrote: >>>> On Mon, Jan 20, 2025 at 07:50:23PM +0100, Simona Vetter wrote: >>>>> On Mon, Jan 20, 2025 at 01:59:01PM -0400, Jason Gunthorpe wrote: >>>>>> On Mon, Jan 20, 2025 at 01:14:12PM +0100, Christian König wrote: >>>>>> What is going wrong with your email? You replied to Simona, but >>>>>> Simona Vetter <simona.vetter(a)ffwll.ch> is dropped from the To/CC >>>>>> list??? I added the address back, but seems like a weird thing to >>>>>> happen. >>>>> Might also be funny mailing list stuff, depending how you get these. I >>>>> read mails over lore and pretty much ignore cc (unless it's not also on >>>>> any list, since those tend to be security issues) because I get cc'ed on >>>>> way too much stuff for that to be a useful signal. >>>> Oh I see, you are sending a Mail-followup-to header that excludes your >>>> address, so you don't get any emails at all.. My mutt is dropping you >>>> as well. I'm having all kind of funny phenomena with AMDs mail servers since coming back from xmas vacation. From the news it looks like Outlook on Windows has a new major security issue where just viewing a mail can compromise the system and my educated guess is that our IT guys went into panic mode because of this and has changed something. >> [SNIP] >> I have been assuming that dmabuf mmap remains unchanged, that >> exporters will continue to implement that mmap() callback as today. That sounds really really good to me because that was my major concern when you noted that you want to have PFNs to build up KVM page tables. But you don't want to handle mmap() on your own, you basically don't want to have a VMA for this stuff at all, correct? >> My main interest has been what data structure is produced in the >> attach APIs. >> >> Eg today we have a struct dma_buf_attachment that returns a sg_table. >> >> I'm expecting some kind of new data structure, lets call it "physical >> list" that is some efficient coding of meta/addr/len tuples that works >> well with the new DMA API. Matthew has been calling this thing phyr.. I would not use a data structure at all. Instead we should have something like an iterator/cursor based approach similar to what the new DMA API is doing. >> So, I imagine, struct dma_buf_attachment gaining an optional >> feature negotiation and then we have in dma_buf_attachment: >> >> union { >> struct sg_table *sgt; >> struct physical_list *phyr; >> }; >> >> That's basicaly it, an alternative to scatterlist that has a clean >> architecture. I would rather suggest something like dma_buf_attachment() gets offset and size to map and returns a cursor object you can use to get your address, length and access attributes. And then you can iterate over this cursor and fill in your importer data structure with the necessary information. This way neither the exporter nor the importer need to convert their data back and forth between their specific representations of the information. >> Now, if you are asking if the current dmabuf mmap callback can be >> improved with the above? Maybe? phyr should have the neccessary >> information inside it to populate a VMA - eventually even fully >> correctly with all the right cachable/encrypted/forbidden/etc flags. That won't work like this. See the exporter needs to be informed about page faults on the VMA to eventually wait for operations to end and sync caches. Otherwise we either potentially allow access to freed up or re-used memory or run into issues with device cache coherency. >> So, you could imagine that exporters could just have one routine to >> generate the phyr list and that goes into the attachment, goes into >> some common code to fill VMA PTEs, and some other common code that >> will convert it into the DMABUF scatterlist. If performance is not a >> concern with these data structure conversions it could be an appealing >> simplification. >> >> And yes, I could imagine the meta information being descriptive enough >> to support the private interconnect cases, the common code could >> detect private meta information and just cleanly fail. > I'm kinda leaning towards entirely separate dma-buf interfaces for the new > phyr stuff, because I fear that adding that to the existing ones will only > make the chaos worse. But that aside sounds all reasonable, and even that > could just be too much worry on my side and mixing phyr into existing > attachments (with a pile of importer/exporter flags probably) is fine. I lean into the other direction. Dmitry and Thomas have done a really good job at cleaning up all the interaction between dynamic and static exporters / importers. Especially that we now have consistent locking for map_dma_buf() and unmap_dma_buf() should make that transition rather straight forward. > For the existing dma-buf importers/exporters I'm kinda hoping for a pure > dma_addr_t based list eventually. Going all the way to a phyr based > approach for everyone might be too much churn, there's some real bad cruft > there. It's not going to work for every case, but it covers a lot of them > and might be less pain for existing importers. The point is we have use cases that won't work without exchanging DMA addresses any more. For example we have cases with multiple devices are in the same IOMMU domain and re-using their DMA address mappings. > But in theory it should be possible to use phyr everywhere eventually, as > long as there's no obviously api-rules-breaking way to go from a phyr back to > a struct page even when that exists. I would rather say we should stick to DMA addresses as much as possible. What we can do is to add an address space description to the addresses, e.g. if it's a PCIe BUS addr in IOMMU domain X, or of it's a device private bus addr or in the case of sharing with iommufd and KVM PFNs. Regards, Christian. >>> At least the device mapping / dma_buf_attachment >>> side should be doable with just the pfn and the new dma-api? >> Yes, that would be my first goal post. Figure out some meta >> information and a container data structure that allows struct >> page-less P2P mapping through the new DMA API. >> >>>> I'm hoping we can get to something where we describe not just how the >>>> pfns should be DMA mapped, but also can describe how they should be >>>> CPU mapped. For instance that this PFN space is always mapped >>>> uncachable, in CPU and in IOMMU. >>> I was pondering whether dma_mmap and friends would be a good place to >>> prototype this and go for a fully generic implementation. But then even >>> those have _wc/_uncached variants. >> Given that the inability to correctly DMA map P2P MMIO without struct >> page is a current pain point and current source of hacks in dmabuf >> exporters, I wanted to make resolving that a priority. >> >> However, if you mean what I described above for "fully generic [dmabuf >> mmap] implementation", then we'd have the phyr datastructure as a >> dependency to attempt that work. >> >> phyr, and particularly the meta information, has a number of >> stakeholders. I was thinking of going first with rdma's memory >> registration flow because we are now pretty close to being able to do >> such a big change, and it can demonstrate most of the requirements. >> >> But that doesn't mean mmap couldn't go concurrently on the same agreed >> datastructure if people are interested. > Yeah cpu mmap needs a lot more, going with a very limited p2p use-case > first only makes sense. > >>>> We also have current bugs in the iommu/vfio side where we are fudging >>>> CC stuff, like assuming CPU memory is encrypted (not always true) and >>>> that MMIO is non-encrypted (not always true) >>> tbf CC pte flags I just don't grok at all. I've once tried to understand >>> what current exporters and gpu drivers do and just gave up. But that's >>> also a bit why I'm worried here because it's an enigma to me. >> For CC, inside the secure world, is some information if each PFN >> inside the VM is 'encrypted' or not. Any VM PTE (including the IOPTEs) >> pointing at the PFN must match the secure world's view of >> 'encrypted'. The VM can ask the secure world to change its view at >> runtime. >> >> The way CC has been bolted on to the kernel so far laregly hides this >> from drivers, so it is troubled to tell in driver code if the PFN you >> have is 'encrypted' or not. Right now the general rule (that is not >> always true) is that struct page CPU memory is encrypted and >> everything else is decrypted. >> >> So right now, you can mostly ignore it and the above assumption >> largely happens for you transparently. >> >> However, soon we will have encrypted P2P MMIO which will stress this >> hiding strategy. > It's already breaking with stuff like virtual gpu drivers, vmwgfx is > fiddling around with these bits (at least last I tried to understand this > all) and I think a few others do too. > >>>>> I thought iommuv2 (or whatever linux calls these) has full fault support >>>>> and could support current move semantics. But yeah for iommu without >>>>> fault support we need some kind of pin or a newly formalized revoke model. >>>> No, this is HW dependent, including PCI device, and I'm aware of no HW >>>> that fully implements this in a way that could be useful to implement >>>> arbitary move semantics for VFIO.. >>> Hm I thought we've had at least prototypes floating around of device fault >>> repair, but I guess that only works with ATS/pasid stuff and not general >>> iommu traffic from devices. Definitely needs some device cooperation since >>> the timeouts of a full fault are almost endless. >> Yes, exactly. What all real devices I'm aware have done is make a >> subset of their traffic work with ATS and PRI, but not all their >> traffic. Without *all* traffic you can't make any generic assumption >> in the iommu that a transient non-present won't be fatal to the >> device. >> >> Stuff like dmabuf move semantics rely on transient non-present being >> non-disruptive... > Ah now I get it, at the iommu level you have to pessimistically assume > whether a device can handle a fault, and none can for all traffic. I was > thinking too much about the driver level where generally the dma-buf you > importer are only used for the subset of device functions that can cope > with faults on many devices. > > Cheers, Sima

5 months

Re: [RFC PATCH 08/12] vfio/pci: Create host unaccessible dma-buf for private device

by Jason Gunthorpe

On Thu, Jan 23, 2025 at 03:41:58PM +0800, Xu Yilun wrote: > I don't have a complete idea yet. But the goal is not to make any > existing driver seamlessly work with secure device. It is to provide a > generic way for bind/attestation/accept, and may save driver's effort > if they don't care about this startup process. There are plenty of > operations that a driver can't do to a secure device, FLR is one of > them. The TDISP SPEC has described some general rules but some are even > device specific. You can FLR a secure device, it just has to be re-secured and re-attested after. Otherwise no VFIO for you. > So I think a driver (including VFIO) expects change to support trusted > device, but may not have to cover bind/attestation/accept flow. I expect changes, but not fundamental ones. VFIO will still have to FLR devices as part of it's security architecture. The entire flow needs to have options for drivers to be involved in the flow, somehow. Jason

5 months

Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

by Jason Gunthorpe

On Wed, Jan 22, 2025 at 12:04:19PM +0100, Simona Vetter wrote: > I'm kinda leaning towards entirely separate dma-buf interfaces for the new > phyr stuff, because I fear that adding that to the existing ones will only > make the chaos worse. Lets see when some patches come up, if importers have to deal with several formats a single attach interface would probably be simpler for them.. > For the existing dma-buf importers/exporters I'm kinda hoping for a pure > dma_addr_t based list eventually. IMHO the least churn would be to have the old importers call a helper (perhaps implicitly in the core code) to convert phyr into a dma mapped scatterlist and then just keep using their existing code exactly as is. Performance improvement would come from importers switching to use the new dma api internally and avoiding the scatterlist allocation, but even the extra alocation is not so bad. Then, perhaps, and I really hesitate to say this, but perhaps to ease the migration we could store a dma mapped list in a phyr using the meta information. That would allow a dmabuf scatterlist exporter to be converted to a phyr with a helper. The importer would have to have a special path to detect the dma mapped mode and skip the normal mapping. The point would be to let maintained drivers use the new data structures and flows easily and still interoperate with older drivers. Otherwise we have to somehow build parallel scatterlist and phyr code flows into importers :\ > Going all the way to a phyr based > approach for everyone might be too much churn, there's some real bad cruft > there. It's not going to work for every case, but it covers a lot of them > and might be less pain for existing importers. I'd hope we can reach every case.. But I don't know what kind of horrors you guys have :) > But in theory it should be possible to use phyr everywhere eventually, as > long as there's no obviously api-rules-breaking way to go from a phyr back to > a struct page even when that exists. I'd say getting a struct page is a perfectly safe operation, from a phyr API perspective. The dmabuf issue seems to be entirely about following the locking rules, an importer is not allowed to get a struct page and switch from reservation locking to page refcount locking. I get the appeal for DRM of blocking struct page use because that directly prevents the above. But, in RDMA I want to re-use the same phyr datastructure for tracking pin_user_pages() CPU memory, and I must get the struct page back so I can put_user_page() it when done. Perhaps we can find some compromise where the phyr data structure has some kind of flag 'disable struct page' and then the phyr_entry_to_page() API will WARN_ON() or something like that. > > However, soon we will have encrypted P2P MMIO which will stress this > > hiding strategy. > > It's already breaking with stuff like virtual gpu drivers, vmwgfx is > fiddling around with these bits (at least last I tried to understand this > all) and I think a few others do too. IMHO, most places I've seen touching this out side arch code feel very hacky. :\ Jason

5 months

Re: [RFC PATCH 08/12] vfio/pci: Create host unaccessible dma-buf for private device

by Jason Gunthorpe

On Wed, Jan 22, 2025 at 12:32:56PM +0800, Xu Yilun wrote: > On Tue, Jan 21, 2025 at 01:43:03PM -0400, Jason Gunthorpe wrote: > > On Tue, Jun 25, 2024 at 05:12:10AM +0800, Xu Yilun wrote: > > > > > When VFIO works as a TEE user in VM, it means an attester (e.g. PCI > > > subsystem) has already moved the device to RUN state. So VFIO & DPDK > > > are all TEE users, no need to manipulate TDISP state between them. > > > AFAICS, this is the most preferred TIO usage in CoCo-VM. > > > > No, unfortunately. Part of the motivation to have the devices be > > unlocked when the VM starts is because there is an expectation that a > > driver in the VM will need to do untrusted operations to boot up the > > I assume these operations are device specific. Yes > > device before it can be switched to the run state. > > > > So any vfio use case needs to imagine that VFIO starts with an > > untrusted device, does stuff to it, then pushes everything through to > > I have concern that VFIO has to do device specific stuff. Our current > expectation is a specific device driver deals with the untrusted > operations, then user writes a 'bind' device sysfs node which detaches > the driver for untrusted, do the attestation and accept, and try match > the driver for trusted (e.g. VFIO). I don't see this as working, VFIO will FLR the device which will destroy anything that was done prior. VFIO itself has to do the sequence and the VFIO userspace has to contain the device specific stuff. The bind/unbind dance for untrusted->trusted would need to be internalized in VFIO without unbinding. The main motivation for the bind/unbind flow was to manage the DMA API, which VFIO does not use. Jason

5 months

Re: [RFC PATCH 08/12] vfio/pci: Create host unaccessible dma-buf for private device

by Jason Gunthorpe

On Tue, Jun 25, 2024 at 05:12:10AM +0800, Xu Yilun wrote: > When VFIO works as a TEE user in VM, it means an attester (e.g. PCI > subsystem) has already moved the device to RUN state. So VFIO & DPDK > are all TEE users, no need to manipulate TDISP state between them. > AFAICS, this is the most preferred TIO usage in CoCo-VM. No, unfortunately. Part of the motivation to have the devices be unlocked when the VM starts is because there is an expectation that a driver in the VM will need to do untrusted operations to boot up the device before it can be switched to the run state. So any vfio use case needs to imagine that VFIO starts with an untrusted device, does stuff to it, then pushes everything through to run. The exact mirror as what a kernel driver should be able to do. How exactly all this very complex stuff works, I have no idea, but this is what I've understood is the target. :\ Jason

5 months, 1 week

Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

by Jason Gunthorpe

On Tue, Jan 21, 2025 at 05:11:32PM +0100, Simona Vetter wrote: > On Mon, Jan 20, 2025 at 03:48:04PM -0400, Jason Gunthorpe wrote: > > On Mon, Jan 20, 2025 at 07:50:23PM +0100, Simona Vetter wrote: > > > On Mon, Jan 20, 2025 at 01:59:01PM -0400, Jason Gunthorpe wrote: > > > > On Mon, Jan 20, 2025 at 01:14:12PM +0100, Christian König wrote: > > > > What is going wrong with your email? You replied to Simona, but > > > > Simona Vetter <simona.vetter(a)ffwll.ch> is dropped from the To/CC > > > > list??? I added the address back, but seems like a weird thing to > > > > happen. > > > > > > Might also be funny mailing list stuff, depending how you get these. I > > > read mails over lore and pretty much ignore cc (unless it's not also on > > > any list, since those tend to be security issues) because I get cc'ed on > > > way too much stuff for that to be a useful signal. > > > > Oh I see, you are sending a Mail-followup-to header that excludes your > > address, so you don't get any emails at all.. My mutt is dropping you > > as well. > > > > > Yeah I'm not worried about cpu mmap locking semantics. drm/ttm is a pretty > > > clear example that you can implement dma-buf mmap with the rules we have, > > > except the unmap_mapping_range might need a bit fudging with a separate > > > address_space. > > > > From my perspective the mmap thing is a bit of a side/DRM-only thing > > as nothing I'm interested in wants to mmap dmabuf into a VMA. > > I guess we could just skip mmap on these pfn exporters, but it also means > a bit more boilerplate. I have been assuming that dmabuf mmap remains unchanged, that exporters will continue to implement that mmap() callback as today. My main interest has been what data structure is produced in the attach APIs. Eg today we have a struct dma_buf_attachment that returns a sg_table. I'm expecting some kind of new data structure, lets call it "physical list" that is some efficient coding of meta/addr/len tuples that works well with the new DMA API. Matthew has been calling this thing phyr.. So, I imagine, struct dma_buf_attachment gaining an optional feature negotiation and then we have in dma_buf_attachment: union { struct sg_table *sgt; struct physical_list *phyr; }; That's basicaly it, an alternative to scatterlist that has a clean architecture. Now, if you are asking if the current dmabuf mmap callback can be improved with the above? Maybe? phyr should have the neccessary information inside it to populate a VMA - eventually even fully correctly with all the right cachable/encrypted/forbidden/etc flags. So, you could imagine that exporters could just have one routine to generate the phyr list and that goes into the attachment, goes into some common code to fill VMA PTEs, and some other common code that will convert it into the DMABUF scatterlist. If performance is not a concern with these data structure conversions it could be an appealing simplification. And yes, I could imagine the meta information being descriptive enough to support the private interconnect cases, the common code could detect private meta information and just cleanly fail. > At least the device mapping / dma_buf_attachment > side should be doable with just the pfn and the new dma-api? Yes, that would be my first goal post. Figure out some meta information and a container data structure that allows struct page-less P2P mapping through the new DMA API. > > I'm hoping we can get to something where we describe not just how the > > pfns should be DMA mapped, but also can describe how they should be > > CPU mapped. For instance that this PFN space is always mapped > > uncachable, in CPU and in IOMMU. > > I was pondering whether dma_mmap and friends would be a good place to > prototype this and go for a fully generic implementation. But then even > those have _wc/_uncached variants. Given that the inability to correctly DMA map P2P MMIO without struct page is a current pain point and current source of hacks in dmabuf exporters, I wanted to make resolving that a priority. However, if you mean what I described above for "fully generic [dmabuf mmap] implementation", then we'd have the phyr datastructure as a dependency to attempt that work. phyr, and particularly the meta information, has a number of stakeholders. I was thinking of going first with rdma's memory registration flow because we are now pretty close to being able to do such a big change, and it can demonstrate most of the requirements. But that doesn't mean mmap couldn't go concurrently on the same agreed datastructure if people are interested. > > We also have current bugs in the iommu/vfio side where we are fudging > > CC stuff, like assuming CPU memory is encrypted (not always true) and > > that MMIO is non-encrypted (not always true) > > tbf CC pte flags I just don't grok at all. I've once tried to understand > what current exporters and gpu drivers do and just gave up. But that's > also a bit why I'm worried here because it's an enigma to me. For CC, inside the secure world, is some information if each PFN inside the VM is 'encrypted' or not. Any VM PTE (including the IOPTEs) pointing at the PFN must match the secure world's view of 'encrypted'. The VM can ask the secure world to change its view at runtime. The way CC has been bolted on to the kernel so far laregly hides this from drivers, so it is troubled to tell in driver code if the PFN you have is 'encrypted' or not. Right now the general rule (that is not always true) is that struct page CPU memory is encrypted and everything else is decrypted. So right now, you can mostly ignore it and the above assumption largely happens for you transparently. However, soon we will have encrypted P2P MMIO which will stress this hiding strategy. > > > I thought iommuv2 (or whatever linux calls these) has full fault support > > > and could support current move semantics. But yeah for iommu without > > > fault support we need some kind of pin or a newly formalized revoke model. > > > > No, this is HW dependent, including PCI device, and I'm aware of no HW > > that fully implements this in a way that could be useful to implement > > arbitary move semantics for VFIO.. > > Hm I thought we've had at least prototypes floating around of device fault > repair, but I guess that only works with ATS/pasid stuff and not general > iommu traffic from devices. Definitely needs some device cooperation since > the timeouts of a full fault are almost endless. Yes, exactly. What all real devices I'm aware have done is make a subset of their traffic work with ATS and PRI, but not all their traffic. Without *all* traffic you can't make any generic assumption in the iommu that a transient non-present won't be fatal to the device. Stuff like dmabuf move semantics rely on transient non-present being non-disruptive... Jason

5 months, 1 week

Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

by Jason Gunthorpe

On Mon, Jan 20, 2025 at 07:50:23PM +0100, Simona Vetter wrote: > On Mon, Jan 20, 2025 at 01:59:01PM -0400, Jason Gunthorpe wrote: > > On Mon, Jan 20, 2025 at 01:14:12PM +0100, Christian König wrote: > > What is going wrong with your email? You replied to Simona, but > > Simona Vetter <simona.vetter(a)ffwll.ch> is dropped from the To/CC > > list??? I added the address back, but seems like a weird thing to > > happen. > > Might also be funny mailing list stuff, depending how you get these. I > read mails over lore and pretty much ignore cc (unless it's not also on > any list, since those tend to be security issues) because I get cc'ed on > way too much stuff for that to be a useful signal. Oh I see, you are sending a Mail-followup-to header that excludes your address, so you don't get any emails at all.. My mutt is dropping you as well. > Yeah I'm not worried about cpu mmap locking semantics. drm/ttm is a pretty > clear example that you can implement dma-buf mmap with the rules we have, > except the unmap_mapping_range might need a bit fudging with a separate > address_space. From my perspective the mmap thing is a bit of a side/DRM-only thing as nothing I'm interested in wants to mmap dmabuf into a VMA. However, I think if you have locking rules that can fit into a VMA fault path and link move_notify to unmap_mapping_range() then you've got a pretty usuable API. > For cpu mmaps I'm more worried about the arch bits in the pte, stuff like > caching mode or encrypted memory bits and things like that. There's > vma->vm_pgprot, but it's a mess. But maybe this all is an incentive to > clean up that mess a bit. I'm convinced we need meta-data along with pfns, there is too much stuff that needs more information than just the address. Cachability, CC encryption, exporting device, etc. This is a topic to partially cross when we talk about how to fully remove struct page requirements from the new DMA API. I'm hoping we can get to something where we describe not just how the pfns should be DMA mapped, but also can describe how they should be CPU mapped. For instance that this PFN space is always mapped uncachable, in CPU and in IOMMU. We also have current bugs in the iommu/vfio side where we are fudging CC stuff, like assuming CPU memory is encrypted (not always true) and that MMIO is non-encrypted (not always true) > I thought iommuv2 (or whatever linux calls these) has full fault support > and could support current move semantics. But yeah for iommu without > fault support we need some kind of pin or a newly formalized revoke model. No, this is HW dependent, including PCI device, and I'm aware of no HW that fully implements this in a way that could be useful to implement arbitary move semantics for VFIO.. Jason

5 months, 1 week

Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI

by Christian König

Am 17.01.25 um 15:42 schrieb Simona Vetter: > On Wed, Jan 15, 2025 at 11:06:53AM +0100, Christian König wrote: >> [SNIP] >>> Anything missing? >> Well as far as I can see this use case is not a good fit for the DMA-buf >> interfaces in the first place. DMA-buf deals with devices and buffer >> exchange. >> >> What's necessary here instead is to give an importing VM full access on some >> memory for their specific use case. >> >> That full access includes CPU and DMA mappings, modifying caching >> attributes, potentially setting encryption keys for specific ranges etc.... >> etc... >> >> In other words we have a lot of things the importer here should be able to >> do which we don't want most of the DMA-buf importers to do. > This proposal isn't about forcing existing exporters to allow importers to > do new stuff. That stays as-is, because it would break things. > > It's about adding yet another interface to get at the underlying data, and > we have tons of those already. The only difference is that if we don't > butcher the design, we'll be able to implement all the existing dma-buf > interfaces on top of this new pfn interface, for some neat maximal > compatibility. That sounds like you missed my concern. When an exporter and an importer agree that they want to exchange PFNs instead of DMA addresses then that is perfectly fine. The problems start when you define the semantics how those PFNs, DMA address, private bus addresses, whatever is echanged different to what we have documented for DMA-buf. This semantics is very well defined for DMA-buf now, because that is really important or otherwise things usually seem to work under testing (e.g. without memory pressure) and then totally fall apart in production environments. In other words we have defined what lock you need to hold when calling functions, what a DMA fence is, when exchanged addresses are valid etc... > But fundamentally there's never been an expectation that you can take any > arbitrary dma-buf and pass it any arbitrary importer, and that is must > work. The fundamental promise is that if it _does_ work, then > - it's zero copy > - and fast, or as fast as we can make it > > I don't see this any different than all the much more specific prposals > and existing code, where a subset of importers/exporters have special > rules so that e.g. gpu interconnect or vfio uuid based sharing works. > pfn-based sharing is just yet another flavor that exists to get the max > amount of speed out of interconnects. Please take another look at what is proposed here. The function is called dma_buf_get_pfn_*unlocked* ! This is not following DMA-buf semantics for exchanging addresses and keeping them valid, but rather something more like userptrs. Inserting PFNs into CPU (or probably also IOMMU) page tables have a different semantics than what DMA-buf usually does, because as soon as the address is written into the page table it is made public. So you need some kind of mechanism to make sure that this addr you made public stays valid as long as it is public. The usual I/O operation we encapsulate with DMA-fences have a fundamentally different semantics because we have the lock which enforces that stuff stays valid and then have a DMA-fence which notes how long the stuff needs to stay valid for an operation to complete. Regards, Christian. > > Cheers, Sima > >> The semantics for things like pin vs revocable vs dynamic/moveable seems >> similar, but that's basically it. >> >> As far as I know the TEE subsystem also represents their allocations as file >> descriptors. If I'm not completely mistaken this use case most likely fit's >> better there. >> >>> I feel like this is small enough that m-l archives is good enough. For >>> some of the bigger projects we do in graphics we sometimes create entries >>> in our kerneldoc with wip design consensus and things like that. But >>> feels like overkill here. >>> >>>> My general desire is to move all of RDMA's MR process away from >>>> scatterlist and work using only the new DMA API. This will save *huge* >>>> amounts of memory in common workloads and be the basis for non-struct >>>> page DMA support, including P2P. >>> Yeah a more memory efficient structure than the scatterlist would be >>> really nice. That would even benefit the very special dma-buf exporters >>> where you cannot get a pfn and only the dma_addr_t, altough most of those >>> (all maybe even?) have contig buffers, so your scatterlist has only one >>> entry. But it would definitely be nice from a design pov. >> Completely agree on that part. >> >> Scatterlist have a some design flaws, especially mixing the input and out >> parameters of the DMA API into the same structure. >> >> Additional to that DMA addresses are basically missing which bus they belong >> to and details how the access should be made (e.g. snoop vs no-snoop >> etc...). >> >>> Aside: A way to more efficiently create compressed scatterlists would be >>> neat too, because a lot of drivers hand-roll that and it's a bit brittle >>> and kinda silly to duplicate. With compressed I mean just a single entry >>> for a contig range, in practice thanks to huge pages/folios and allocators >>> trying to hand out contig ranges if there's plenty of memory that saves a >>> lot of memory too. But currently it's a bit a pain to construct these >>> efficiently, mostly it's just a two-pass approach and then trying to free >>> surplus memory or krealloc to fit. Also I don't have good ideas here, but >>> dma-api folks might have some from looking at too many things that create >>> scatterlists. >> I mailed with Christoph about that a while back as well and we both agreed >> that it would probably be a good idea to start defining a data structure to >> better encapsulate DMA addresses. >> >> It's just that nobody had time for that yet and/or I wasn't looped in in the >> final discussion about it. >> >> Regards, >> Christian. >> >>> -Sima

5 months, 1 week

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