On Fri, Jun 18, 2021 at 2:36 PM Oded Gabbay ogabbay@kernel.org wrote:
User process might want to share the device memory with another driver/device, and to allow it to access it over PCIe (P2P).
To enable this, we utilize the dma-buf mechanism and add a dma-buf exporter support, so the other driver can import the device memory and access it.
The device memory is allocated using our existing allocation uAPI, where the user will get a handle that represents the allocation.
The user will then need to call the new uAPI (HL_MEM_OP_EXPORT_DMABUF_FD) and give the handle as a parameter.
The driver will return a FD that represents the DMA-BUF object that was created to match that allocation.
Signed-off-by: Oded Gabbay ogabbay@kernel.org Reviewed-by: Tomer Tayar ttayar@habana.ai
Mission acomplished, we've gone full circle, and the totally-not-a-gpu driver is now trying to use gpu infrastructure. And seems to have gained vram meanwhile too. Next up is going to be synchronization using dma_fence so you can pass buffers back&forth without stalls among drivers.
Bonus points for this being at v3 before it shows up on dri-devel and cc's dma-buf folks properly (not quite all, I added the missing people).
I think we roughly have two options here
a) Greg continues to piss off dri-devel folks while trying to look cute&cuddly and steadfastly claiming that this accelator doesn't work like any of the other accelerator drivers we have in drivers/gpu/drm. All while the driver ever more looks like one of these other accel drivers.
b) We finally do what we should have done years back and treat this as a proper driver submission and review it on dri-devel instead of sneaking it in through other channels because the merge criteria dri-devel has are too onerous and people who don't have experience with accel stacks for the past 20 years or so don't like them.
"But this probably means a new driver and big disruption!"
Not my problem, I'm not the dude who has to come up with an excuse for this because I didn't merge the driver in the first place. I do get to throw a "we all told you so" in though, but that's not helping.
Also I'm wondering which is the other driver that we share buffers with. The gaudi stuff doesn't have real struct pages as backing storage, it only fills out the dma_addr_t. That tends to blow up with other drivers, and the only place where this is guaranteed to work is if you have a dynamic importer which sets the allow_peer2peer flag. Adding maintainers from other subsystems who might want to chime in here. So even aside of the big question as-is this is broken.
Currently only 2 drivers set allow_peer2peer, so those are the only ones who can consume these buffers from device memory. Pinging those folks specifically.
Doug/Jason from infiniband: Should we add linux-rdma to the dma-buf wildcard match so that you can catch these next time around too? At least when people use scripts/get_maintainers.pl correctly. All the other subsystems using dma-buf are on there already (dri-devel, linux-media and linaro-mm-sig for android/arm embedded stuff).
Cheers, Daniel
include/uapi/misc/habanalabs.h | 28 +++++++++++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-)
diff --git a/include/uapi/misc/habanalabs.h b/include/uapi/misc/habanalabs.h index a47a731e4527..aa3d8e0ba060 100644 --- a/include/uapi/misc/habanalabs.h +++ b/include/uapi/misc/habanalabs.h @@ -808,6 +808,10 @@ union hl_wait_cs_args { #define HL_MEM_OP_UNMAP 3 /* Opcode to map a hw block */ #define HL_MEM_OP_MAP_BLOCK 4 +/* Opcode to create DMA-BUF object for an existing device memory allocation
- and to export an FD of that DMA-BUF back to the caller
- */
+#define HL_MEM_OP_EXPORT_DMABUF_FD 5
/* Memory flags */ #define HL_MEM_CONTIGUOUS 0x1 @@ -878,11 +882,26 @@ struct hl_mem_in { /* Virtual address returned from HL_MEM_OP_MAP */ __u64 device_virt_addr; } unmap;
/* HL_MEM_OP_EXPORT_DMABUF_FD */struct {/* Handle returned from HL_MEM_OP_ALLOC. In Gaudi,* where we don't have MMU for the device memory, the* driver expects a physical address (instead of* a handle) in the device memory space.*/__u64 handle;/* Size of memory allocation. Relevant only for GAUDI */__u64 mem_size;} export_dmabuf_fd; }; /* HL_MEM_OP_* */ __u32 op;
/* HL_MEM_* flags */
/* HL_MEM_* flags.* For the HL_MEM_OP_EXPORT_DMABUF_FD opcode, this field holds the* DMA-BUF file/FD flags.*/ __u32 flags; /* Context ID - Currently not in use */ __u32 ctx_id;@@ -919,6 +938,13 @@ struct hl_mem_out {
__u32 pad; };
/* Returned in HL_MEM_OP_EXPORT_DMABUF_FD. Represents the* DMA-BUF object that was created to describe a memory* allocation on the device's memory space. The FD should be* passed to the importer driver*/__u64 fd; };};
-- 2.25.1
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
On Mon, Jun 21, 2021 at 02:28:48PM +0200, Daniel Vetter wrote:
On Fri, Jun 18, 2021 at 2:36 PM Oded Gabbay ogabbay@kernel.org wrote:
User process might want to share the device memory with another driver/device, and to allow it to access it over PCIe (P2P).
To enable this, we utilize the dma-buf mechanism and add a dma-buf exporter support, so the other driver can import the device memory and access it.
The device memory is allocated using our existing allocation uAPI, where the user will get a handle that represents the allocation.
The user will then need to call the new uAPI (HL_MEM_OP_EXPORT_DMABUF_FD) and give the handle as a parameter.
The driver will return a FD that represents the DMA-BUF object that was created to match that allocation.
Signed-off-by: Oded Gabbay ogabbay@kernel.org Reviewed-by: Tomer Tayar ttayar@habana.ai
Mission acomplished, we've gone full circle, and the totally-not-a-gpu driver is now trying to use gpu infrastructure. And seems to have gained vram meanwhile too. Next up is going to be synchronization using dma_fence so you can pass buffers back&forth without stalls among drivers.
What's wrong with other drivers using dmabufs and even dma_fence? It's a common problem when shuffling memory around systems, why is that somehow only allowed for gpu drivers?
There are many users of these structures in the kernel today that are not gpu drivers (tee, fastrpc, virtio, xen, IB, etc) as this is a common thing that drivers want to do (throw chunks of memory around from userspace to hardware).
I'm not trying to be a pain here, but I really do not understand why this is a problem. A kernel api is present, why not use it by other in-kernel drivers? We had the problem in the past where subsystems were trying to create their own interfaces for the same thing, which is why you all created the dmabuf api to help unify this.
Also I'm wondering which is the other driver that we share buffers with. The gaudi stuff doesn't have real struct pages as backing storage, it only fills out the dma_addr_t. That tends to blow up with other drivers, and the only place where this is guaranteed to work is if you have a dynamic importer which sets the allow_peer2peer flag. Adding maintainers from other subsystems who might want to chime in here. So even aside of the big question as-is this is broken.
From what I can tell this driver is sending the buffers to other
instances of the same hardware, as that's what is on the other "end" of the network connection. No different from IB's use of RDMA, right?
thanks,
greg k-h
On Mon, Jun 21, 2021 at 03:02:10PM +0200, Greg KH wrote:
On Mon, Jun 21, 2021 at 02:28:48PM +0200, Daniel Vetter wrote:
Also I'm wondering which is the other driver that we share buffers with. The gaudi stuff doesn't have real struct pages as backing storage, it only fills out the dma_addr_t. That tends to blow up with other drivers, and the only place where this is guaranteed to work is if you have a dynamic importer which sets the allow_peer2peer flag. Adding maintainers from other subsystems who might want to chime in here. So even aside of the big question as-is this is broken.
From what I can tell this driver is sending the buffers to other instances of the same hardware,
A dmabuf is consumed by something else in the kernel calling dma_buf_map_attachment() on the FD.
What is the other side of this? I don't see any dma_buf_map_attachment() calls in drivers/misc, or added in this patch set.
AFAIK the only viable in-tree other side is in mlx5 (look in umem_dmabuf.c)
Though as we already talked habana has their own networking (out of tree, presumably) so I suspect this is really to support some out of tree stuff??
Jason
On Mon, Jun 21, 2021 at 03:02:10PM +0200, Greg KH wrote:
On Mon, Jun 21, 2021 at 02:28:48PM +0200, Daniel Vetter wrote:
On Fri, Jun 18, 2021 at 2:36 PM Oded Gabbay ogabbay@kernel.org wrote:
User process might want to share the device memory with another driver/device, and to allow it to access it over PCIe (P2P).
To enable this, we utilize the dma-buf mechanism and add a dma-buf exporter support, so the other driver can import the device memory and access it.
The device memory is allocated using our existing allocation uAPI, where the user will get a handle that represents the allocation.
The user will then need to call the new uAPI (HL_MEM_OP_EXPORT_DMABUF_FD) and give the handle as a parameter.
The driver will return a FD that represents the DMA-BUF object that was created to match that allocation.
Signed-off-by: Oded Gabbay ogabbay@kernel.org Reviewed-by: Tomer Tayar ttayar@habana.ai
Mission acomplished, we've gone full circle, and the totally-not-a-gpu driver is now trying to use gpu infrastructure. And seems to have gained vram meanwhile too. Next up is going to be synchronization using dma_fence so you can pass buffers back&forth without stalls among drivers.
What's wrong with other drivers using dmabufs and even dma_fence? It's a common problem when shuffling memory around systems, why is that somehow only allowed for gpu drivers?
There are many users of these structures in the kernel today that are not gpu drivers (tee, fastrpc, virtio, xen, IB, etc) as this is a common thing that drivers want to do (throw chunks of memory around from userspace to hardware).
I'm not trying to be a pain here, but I really do not understand why this is a problem. A kernel api is present, why not use it by other in-kernel drivers? We had the problem in the past where subsystems were trying to create their own interfaces for the same thing, which is why you all created the dmabuf api to help unify this.
It's the same thing as ever. 90% of an accel driver are in userspace, that's where all the fun is, that's where the big picture review needs to happen, and we've very conveniently bypassed all that a few years back because it was too annoying.
Once we have the full driver stack and can start reviewing it I have no objections to totally-not-gpus using all this stuff too. But until we can do that this is all just causing headaches.
Ofc if you assume that userspace doesn't matter then you don't care, which is where this giantic disconnect comes from.
Also unless we're actually doing this properly there's zero incentive for me to review the kernel code and check whether it follows the rules correctly, so you have excellent chances that you just break the rules. And dma_buf/fence are tricky enough that you pretty much guaranteed to break the rules if you're not involved in the discussions. Just now we have a big one where everyone involved (who's been doing this for 10+ years all at least) realizes we've fucked up big time.
Anyway we've had this discussion, we're not going to move anyone here at all, so *shrug*. I'll keep seeing accelarators in drivers/misc as blantant bypassing of review by actual accelerator pieces, you keep seing dri-devel as ... well I dunno, people who don't know what they're talking about maybe. Or not relevant to your totally-not-a-gpu thing.
Also I'm wondering which is the other driver that we share buffers with. The gaudi stuff doesn't have real struct pages as backing storage, it only fills out the dma_addr_t. That tends to blow up with other drivers, and the only place where this is guaranteed to work is if you have a dynamic importer which sets the allow_peer2peer flag. Adding maintainers from other subsystems who might want to chime in here. So even aside of the big question as-is this is broken.
From what I can tell this driver is sending the buffers to other instances of the same hardware, as that's what is on the other "end" of the network connection. No different from IB's use of RDMA, right?
There's no import afaict, but maybe I missed it. Assuming I haven't missed it the importing necessarily has to happen by some other drivers. -Daniel
On Mon, Jun 21, 2021 at 04:20:35PM +0200, Daniel Vetter wrote:
Also unless we're actually doing this properly there's zero incentive for me to review the kernel code and check whether it follows the rules correctly, so you have excellent chances that you just break the rules. And dma_buf/fence are tricky enough that you pretty much guaranteed to break the rules if you're not involved in the discussions. Just now we have a big one where everyone involved (who's been doing this for 10+ years all at least) realizes we've fucked up big time.
This is where I come from on dmabuf, it is fiendishly complicated. Don't use it unless you absoultely have to, are in DRM, and have people like Daniel helping to make sure you use it right.
It's whole premise and design is compromised by specialty historical implementation choices on the GPU side.
Jason
On Mon, Jun 21, 2021 at 02:28:48PM +0200, Daniel Vetter wrote:
Mission acomplished, we've gone full circle, and the totally-not-a-gpu driver is now trying to use gpu infrastructure. And seems to have gained vram meanwhile too. Next up is going to be synchronization using dma_fence so you can pass buffers back&forth without stalls among drivers.
Well, we can't even see the other side of this so who knows
This is a new uAPI, where is the userspace? In RDMA at least I require to see the new userspace and test suite before changes to include/uapi/rdma can go ahead.
Doug/Jason from infiniband: Should we add linux-rdma to the dma-buf wildcard match so that you can catch these next time around too? At least when people use scripts/get_maintainers.pl correctly. All the other subsystems using dma-buf are on there already (dri-devel, linux-media and linaro-mm-sig for android/arm embedded stuff).
My bigger concern is this doesn't seem to be implementing PCI P2P DMA correctly. This is following the same hacky NULL page approach that Christoph Hellwig already NAK'd for AMD.
This should not be allowed to proliferate.
I would be much happier seeing this be done using the approach of Logan's series here:
https://lore.kernel.org/linux-block/20210513223203.5542-1-logang@deltatee.co...
Jason
linaro-mm-sig@lists.linaro.org
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