On 23.03.23 23:50, Sean Christopherson wrote:
On Wed, 22 Mar 2023 02:37:25 +0100, Mathias Krause wrote:
v3: https://lore.kernel.org/kvm/20230201194604.11135-1-minipli@grsecurity.net/
This series is the fourth iteration of resurrecting the missing pieces of Paolo's previous attempt[1] to avoid needless MMU roots unloading.
It's incorporating Sean's feedback to v3 and rebased on top of kvm-x86/next, namely commit d8708b80fa0e ("KVM: Change return type of kvm_arch_vm_ioctl() to "int"").
[...]
Applied 1 and 5 to kvm-x86 mmu, and the rest to kvm-x86 misc, thanks!
[1/6] KVM: x86/mmu: Avoid indirect call for get_cr3 https://github.com/kvm-x86/linux/commit/2fdcc1b32418 [2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled https://github.com/kvm-x86/linux/commit/01b31714bd90 [3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode https://github.com/kvm-x86/linux/commit/e40bcf9f3a18 [4/6] KVM: x86: Make use of kvm_read_cr*_bits() when testing bits https://github.com/kvm-x86/linux/commit/74cdc836919b [5/6] KVM: x86/mmu: Fix comment typo https://github.com/kvm-x86/linux/commit/50f13998451e [6/6] KVM: VMX: Make CR0.WP a guest owned bit https://github.com/kvm-x86/linux/commit/fb509f76acc8
Thanks a lot, Sean!
As this is a huge performance fix for us, we'd like to get it integrated into current stable kernels as well -- not without having the changes get some wider testing, of course, i.e. not before they end up in a non-rc version released by Linus. But I already did a backport to 5.4 to get a feeling how hard it would be and for the impact it has on older kernels.
Using the 'ssdd 10 50000' test I used before, I get promising results there as well. Without the patches it takes 9.31s, while with them we're down to 4.64s. Taking into account that this is the runtime of a workload in a VM that gets cut in half, I hope this qualifies as stable material, as it's a huge performance fix.
Greg, what's your opinion on it? Original series here: https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@grsecurity.net/
Thanks, Mathias
On Sat, Mar 25, 2023 at 12:39:59PM +0100, Mathias Krause wrote:
On 23.03.23 23:50, Sean Christopherson wrote:
On Wed, 22 Mar 2023 02:37:25 +0100, Mathias Krause wrote:
v3: https://lore.kernel.org/kvm/20230201194604.11135-1-minipli@grsecurity.net/
This series is the fourth iteration of resurrecting the missing pieces of Paolo's previous attempt[1] to avoid needless MMU roots unloading.
It's incorporating Sean's feedback to v3 and rebased on top of kvm-x86/next, namely commit d8708b80fa0e ("KVM: Change return type of kvm_arch_vm_ioctl() to "int"").
[...]
Applied 1 and 5 to kvm-x86 mmu, and the rest to kvm-x86 misc, thanks!
[1/6] KVM: x86/mmu: Avoid indirect call for get_cr3 https://github.com/kvm-x86/linux/commit/2fdcc1b32418 [2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled https://github.com/kvm-x86/linux/commit/01b31714bd90 [3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode https://github.com/kvm-x86/linux/commit/e40bcf9f3a18 [4/6] KVM: x86: Make use of kvm_read_cr*_bits() when testing bits https://github.com/kvm-x86/linux/commit/74cdc836919b [5/6] KVM: x86/mmu: Fix comment typo https://github.com/kvm-x86/linux/commit/50f13998451e [6/6] KVM: VMX: Make CR0.WP a guest owned bit https://github.com/kvm-x86/linux/commit/fb509f76acc8
Thanks a lot, Sean!
As this is a huge performance fix for us, we'd like to get it integrated into current stable kernels as well -- not without having the changes get some wider testing, of course, i.e. not before they end up in a non-rc version released by Linus. But I already did a backport to 5.4 to get a feeling how hard it would be and for the impact it has on older kernels.
Using the 'ssdd 10 50000' test I used before, I get promising results there as well. Without the patches it takes 9.31s, while with them we're down to 4.64s. Taking into account that this is the runtime of a workload in a VM that gets cut in half, I hope this qualifies as stable material, as it's a huge performance fix.
Greg, what's your opinion on it? Original series here: https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@grsecurity.net/
I'll leave the judgement call up to the KVM maintainers, as they are the ones that need to ack any KVM patch added to stable trees.
thanks,
greg k-h
On Sat, Mar 25, 2023, Greg KH wrote:
On Sat, Mar 25, 2023 at 12:39:59PM +0100, Mathias Krause wrote:
On 23.03.23 23:50, Sean Christopherson wrote:
On Wed, 22 Mar 2023 02:37:25 +0100, Mathias Krause wrote:
v3: https://lore.kernel.org/kvm/20230201194604.11135-1-minipli@grsecurity.net/
This series is the fourth iteration of resurrecting the missing pieces of Paolo's previous attempt[1] to avoid needless MMU roots unloading.
It's incorporating Sean's feedback to v3 and rebased on top of kvm-x86/next, namely commit d8708b80fa0e ("KVM: Change return type of kvm_arch_vm_ioctl() to "int"").
[...]
Applied 1 and 5 to kvm-x86 mmu, and the rest to kvm-x86 misc, thanks!
[1/6] KVM: x86/mmu: Avoid indirect call for get_cr3 https://github.com/kvm-x86/linux/commit/2fdcc1b32418 [2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled https://github.com/kvm-x86/linux/commit/01b31714bd90 [3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode https://github.com/kvm-x86/linux/commit/e40bcf9f3a18 [4/6] KVM: x86: Make use of kvm_read_cr*_bits() when testing bits https://github.com/kvm-x86/linux/commit/74cdc836919b [5/6] KVM: x86/mmu: Fix comment typo https://github.com/kvm-x86/linux/commit/50f13998451e [6/6] KVM: VMX: Make CR0.WP a guest owned bit https://github.com/kvm-x86/linux/commit/fb509f76acc8
Thanks a lot, Sean!
As this is a huge performance fix for us, we'd like to get it integrated into current stable kernels as well -- not without having the changes get some wider testing, of course, i.e. not before they end up in a non-rc version released by Linus. But I already did a backport to 5.4 to get a feeling how hard it would be and for the impact it has on older kernels.
Using the 'ssdd 10 50000' test I used before, I get promising results there as well. Without the patches it takes 9.31s, while with them we're down to 4.64s. Taking into account that this is the runtime of a workload in a VM that gets cut in half, I hope this qualifies as stable material, as it's a huge performance fix.
Greg, what's your opinion on it? Original series here: https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@grsecurity.net/
I'll leave the judgement call up to the KVM maintainers, as they are the ones that need to ack any KVM patch added to stable trees.
These are quite risky to backport. E.g. we botched patch 6[*], and my initial fix also had a subtle bug. There have also been quite a few KVM MMU changes since 5.4, so it's possible that an edge case may exist in 5.4 that doesn't exist in mainline.
I'm not totally opposed to the idea since our tests _should_ be provide solid coverage, e.g. existing tests caught my subtle bug, but I don't think we should backport these without a solid usecase, as there is a fairly high risk of breaking random KVM users that wouldn't see any meaningful benefit.
In other words, who cares enough about the performance of running grsecurity kernels in VMs to want these backported, but doesn't have the resources to maintain (or pay someone to maintain) their own host kernel?
[*] https://lkml.kernel.org/r/20230405002608.418442-1-seanjc%40google.com
On 06.04.23 04:25, Sean Christopherson wrote:
On Sat, Mar 25, 2023, Greg KH wrote:
On Sat, Mar 25, 2023 at 12:39:59PM +0100, Mathias Krause wrote:
As this is a huge performance fix for us, we'd like to get it integrated into current stable kernels as well -- not without having the changes get some wider testing, of course, i.e. not before they end up in a non-rc version released by Linus. But I already did a backport to 5.4 to get a feeling how hard it would be and for the impact it has on older kernels.
Using the 'ssdd 10 50000' test I used before, I get promising results there as well. Without the patches it takes 9.31s, while with them we're down to 4.64s. Taking into account that this is the runtime of a workload in a VM that gets cut in half, I hope this qualifies as stable material, as it's a huge performance fix.
Greg, what's your opinion on it? Original series here: https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@grsecurity.net/
I'll leave the judgement call up to the KVM maintainers, as they are the ones that need to ack any KVM patch added to stable trees.
These are quite risky to backport. E.g. we botched patch 6[*], and my initial fix also had a subtle bug. There have also been quite a few KVM MMU changes since 5.4, so it's possible that an edge case may exist in 5.4 that doesn't exist in mainline.
I totally agree. Getting the changes to work with older kernels needs more work. The MMU role handling was refactored in 5.14 and down to 5.4 it differs even more, so backports to earlier kernels definitely needs more care.
My plan would be to limit backporting of the whole series to kernels down to 5.15 (maybe 5.10 if it turns out to be doable) and for kernels before that only without patch 6. That would leave out the problematic change but still give us the benefits of dropping the needless mmu unloads for only toggling CR0.WP in the VM. This already helps us a lot!
I'm not totally opposed to the idea since our tests _should_ be provide solid coverage, e.g. existing tests caught my subtle bug, but I don't think we should backport these without a solid usecase, as there is a fairly high risk of breaking random KVM users that wouldn't see any meaningful benefit.
In other words, who cares enough about the performance of running grsecurity kernels in VMs to want these backported, but doesn't have the resources to maintain (or pay someone to maintain) their own host kernel?
The ones who care are, obviously, our customers -- and we, of course! Customers that can run their own infrastructure don't need these backports in upstream LTS kernels, as we will provide them as well. However, customers that rent VMs in the cloud have no control of what runs as host kernel. It'll likely be some distribution kernel or some tailored version of that, which is likely based on one of the LTS kernels.
Proxmox[1], for example, is a Debian based virtualization management system. They do provide their own kernels, based on 5.15. However, the official Debian stable kernel is based on 5.10. So it would be nice to get backports down to this version at least.
[1] https://www.proxmox.com/en/proxmox-ve/features
[*] https://lkml.kernel.org/r/20230405002608.418442-1-seanjc%40google.com
On 06.04.23 15:22, Mathias Krause wrote:
On 06.04.23 04:25, Sean Christopherson wrote:
On Sat, Mar 25, 2023, Greg KH wrote:
On Sat, Mar 25, 2023 at 12:39:59PM +0100, Mathias Krause wrote:
As this is a huge performance fix for us, we'd like to get it integrated into current stable kernels as well -- not without having the changes get some wider testing, of course, i.e. not before they end up in a non-rc version released by Linus. But I already did a backport to 5.4 to get a feeling how hard it would be and for the impact it has on older kernels.
Using the 'ssdd 10 50000' test I used before, I get promising results there as well. Without the patches it takes 9.31s, while with them we're down to 4.64s. Taking into account that this is the runtime of a workload in a VM that gets cut in half, I hope this qualifies as stable material, as it's a huge performance fix.
Greg, what's your opinion on it? Original series here: https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@grsecurity.net/
I'll leave the judgement call up to the KVM maintainers, as they are the ones that need to ack any KVM patch added to stable trees.
These are quite risky to backport. E.g. we botched patch 6[*], and my initial fix also had a subtle bug. There have also been quite a few KVM MMU changes since 5.4, so it's possible that an edge case may exist in 5.4 that doesn't exist in mainline.
I totally agree. Getting the changes to work with older kernels needs more work. The MMU role handling was refactored in 5.14 and down to 5.4 it differs even more, so backports to earlier kernels definitely needs more care.
My plan would be to limit backporting of the whole series to kernels down to 5.15 (maybe 5.10 if it turns out to be doable) and for kernels before that only without patch 6. That would leave out the problematic change but still give us the benefits of dropping the needless mmu unloads for only toggling CR0.WP in the VM. This already helps us a lot!
To back up the "helps us a lot" with some numbers, here are the results I got from running the 'ssdd 10 50000' micro-benchmark on the backports I did, running on a grsecurity L1 VM (host is a vanilla kernel, as stated below; runtime in seconds, lower is better):
legacy TDP shadow Linux v5.4.240 - 8.87s 56.8s + patches - 5.84s 55.4s
Linux v5.10.177 10.37s 88.7s 69.7s + patches 4.88s 4.92s 70.1s
Linux v5.15.106 9.94s 66.1s 64.9s + patches 4.81s 4.79s 64.6s
Linux v6.1.23 7.65s 8.23s 68.7s + patches 3.36s 3.36s 69.1s
Linux v6.2.10 7.61s 7.98s 68.6s + patches 3.37s 3.41s 70.2s
I guess we can grossly ignore the shadow MMU numbers, beside noting them to regress from v5.4 to v5.10 (something to investigate?). The backports don't help (much) for shadow MMU setups and the flux in the measurements is likely related to the slab allocations involved.
Another unrelated data point is that TDP MMU is really broken for our use case on v5.10 and v5.15 -- it's even slower that shadow paging!
OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU on v5.10.
I backported the whole series down to v5.10 but left out the CR0.WP guest owning patch+fix for v5.4 as the code base is too different to get all the nuances right, as Sean already hinted. However, even this limited backport provides a big performance fix for our use case!
Thanks, Mathias
+Jeremi
On Fri, Apr 14, 2023, Mathias Krause wrote:
On 06.04.23 15:22, Mathias Krause wrote:
On 06.04.23 04:25, Sean Christopherson wrote:
These are quite risky to backport. E.g. we botched patch 6[*], and my initial fix also had a subtle bug. There have also been quite a few KVM MMU changes since 5.4, so it's possible that an edge case may exist in 5.4 that doesn't exist in mainline.
I totally agree. Getting the changes to work with older kernels needs more work. The MMU role handling was refactored in 5.14 and down to 5.4 it differs even more, so backports to earlier kernels definitely needs more care.
My plan would be to limit backporting of the whole series to kernels down to 5.15 (maybe 5.10 if it turns out to be doable) and for kernels before that only without patch 6. That would leave out the problematic change but still give us the benefits of dropping the needless mmu unloads for only toggling CR0.WP in the VM. This already helps us a lot!
To back up the "helps us a lot" with some numbers, here are the results I got from running the 'ssdd 10 50000' micro-benchmark on the backports I did, running on a grsecurity L1 VM (host is a vanilla kernel, as stated below; runtime in seconds, lower is better):
legacy TDP shadow Linux v5.4.240 - 8.87s 56.8s + patches - 5.84s 55.4s
I believe "legacy" and "TDP" are flip-flopped, the TDP MMU does't exist in v5.4.
Linux v5.10.177 10.37s 88.7s 69.7s + patches 4.88s 4.92s 70.1s Linux v5.15.106 9.94s 66.1s 64.9s + patches 4.81s 4.79s 64.6s Linux v6.1.23 7.65s 8.23s 68.7s + patches 3.36s 3.36s 69.1s Linux v6.2.10 7.61s 7.98s 68.6s + patches 3.37s 3.41s 70.2sI guess we can grossly ignore the shadow MMU numbers, beside noting them to regress from v5.4 to v5.10 (something to investigate?). The backports don't help (much) for shadow MMU setups and the flux in the measurements is likely related to the slab allocations involved.
Another unrelated data point is that TDP MMU is really broken for our use case on v5.10 and v5.15 -- it's even slower that shadow paging!
OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU on v5.10.
Anyone that's enabling the TDP MMU on v5.10 is on their own, we didn't enable the TDP MMU by default until v5.14 for very good reasons.
I backported the whole series down to v5.10 but left out the CR0.WP guest owning patch+fix for v5.4 as the code base is too different to get all the nuances right, as Sean already hinted. However, even this limited backport provides a big performance fix for our use case!
As a compromise of sorts, I propose that we disable the TDP MMU by default on v5.15, and backport these fixes to v6.1. v5.15 and earlier won't get "ludicrous speed", but I think that's perfectly acceptable since KVM has had the suboptimal behavior literally since EPT/NPT support was first added.
I'm comfortable backporting to v6.1 as that is recent enough, and there weren't substantial MMU changes between v6.1 and v6.3 in this area. I.e. I have a decent level of confidence that we aren't overlooking some subtle dependency.
For v5.15, I am less confident in the safety of a backport, and more importantly, I think we should disable the TDP MMU by default to mitigate the underlying flaw that makes the 18x speedup possible. That flaw is that KVM can end up freeing and rebuilding TDP MMU roots every time CR0.WP is toggled or a vCPU transitions to/from SMM.
We mitigated the CR0.WP case between v5.15 and v6.1[1], which is why v6.1 doesn't exhibit the same pain as v5.10, but Jeremi discovered that the SMM case badly affects KVM-on-HyperV[2], e.g. when lauching KVM guests using WSL. I posted a fix[3] to finally resolve the underlying bug, but as Jeremi discovered[4], backporting the fix to v5.15 is going to be gnarly, to say the least. It'll be far worse than backporting these CR0.WP patches, and maybe even infeasible without a large scale rework (no thanks).
Anyone that will realize meaningful benefits from the TDP MMU is all but guaranteed to be rolling their own kernels, i.e. can do the backports themselves if they want to use a v5.15 based kernel. The big selling point of the TDP MMU is that it scales better to hundreds of vCPUs, particularly when live migrating such VMs. I highly doubt that anyone running a stock kernel is running 100+ vCPU VMs, let alone trying to live migrate them.
[1] https://lkml.kernel.org/r/20220209170020.1775368-1-pbonzini%40redhat.com [2] https://lore.kernel.org/all/959c5bce-beb5-b463-7158-33fc4a4f910c@linux.micro... [3] https://lore.kernel.org/all/20230413231251.1481410-1-seanjc@google.com [4] https://lore.kernel.org/all/7332d846-fada-eb5c-6068-18ff267bd37f@linux.micro...
On Fri, Apr 14, 2023 at 09:49:28AM -0700, Sean Christopherson wrote:
+Jeremi
Adding myself :)
On Fri, Apr 14, 2023, Mathias Krause wrote:
...
OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU on v5.10.
Anyone that's enabling the TDP MMU on v5.10 is on their own, we didn't enable the TDP MMU by default until v5.14 for very good reasons.
I backported the whole series down to v5.10 but left out the CR0.WP guest owning patch+fix for v5.4 as the code base is too different to get all the nuances right, as Sean already hinted. However, even this limited backport provides a big performance fix for our use case!
As a compromise of sorts, I propose that we disable the TDP MMU by default on v5.15, and backport these fixes to v6.1. v5.15 and earlier won't get "ludicrous speed", but I think that's perfectly acceptable since KVM has had the suboptimal behavior literally since EPT/NPT support was first added.
Disabling TDP MMU for v5.15, and backporting things to v6.1 works for me.
I'm comfortable backporting to v6.1 as that is recent enough, and there weren't substantial MMU changes between v6.1 and v6.3 in this area. I.e. I have a decent level of confidence that we aren't overlooking some subtle dependency.
For v5.15, I am less confident in the safety of a backport, and more importantly, I think we should disable the TDP MMU by default to mitigate the underlying flaw that makes the 18x speedup possible. That flaw is that KVM can end up freeing and rebuilding TDP MMU roots every time CR0.WP is toggled or a vCPU transitions to/from SMM.
The interesting thing here is that these CR0.WP fixes seem to improve things with legacy MMU as well, and legacy MMU is not affected/touched by [3].
So I think you can consider Mathias' ask independent of disabling TDP MMU. On the one hand: there is no regression here. On the other: the gain is big and seems important to him.
I didn't have time to review these patches so I can't judge risk-benefit, or whether any single patch might be a silver bullet on its own.
We mitigated the CR0.WP case between v5.15 and v6.1[1], which is why v6.1 doesn't exhibit the same pain as v5.10, but Jeremi discovered that the SMM case badly affects KVM-on-HyperV[2], e.g. when lauching KVM guests using WSL. I posted a fix[3] to finally resolve the underlying bug, but as Jeremi discovered[4], backporting the fix to v5.15 is going to be gnarly, to say the least. It'll be far worse than backporting these CR0.WP patches, and maybe even infeasible without a large scale rework (no thanks).
Anyone that will realize meaningful benefits from the TDP MMU is all but guaranteed to be rolling their own kernels, i.e. can do the backports themselves if they want to use a v5.15 based kernel. The big selling point of the TDP MMU is that it scales better to hundreds of vCPUs, particularly when live migrating such VMs. I highly doubt that anyone running a stock kernel is running 100+ vCPU VMs, let alone trying to live migrate them.
[1] https://lkml.kernel.org/r/20220209170020.1775368-1-pbonzini%40redhat.com [2] https://lore.kernel.org/all/959c5bce-beb5-b463-7158-33fc4a4f910c@linux.micro... [3] https://lore.kernel.org/all/20230413231251.1481410-1-seanjc@google.com [4] https://lore.kernel.org/all/7332d846-fada-eb5c-6068-18ff267bd37f@linux.micro...
On Fri, Apr 14, 2023, Jeremi Piotrowski wrote:
On Fri, Apr 14, 2023 at 09:49:28AM -0700, Sean Christopherson wrote:
+Jeremi
Adding myself :)
/facepalm
This isn't some mundane detail, Michael!!!
On Fri, Apr 14, 2023, Mathias Krause wrote:
...
OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU on v5.10.
Anyone that's enabling the TDP MMU on v5.10 is on their own, we didn't enable the TDP MMU by default until v5.14 for very good reasons.
I backported the whole series down to v5.10 but left out the CR0.WP guest owning patch+fix for v5.4 as the code base is too different to get all the nuances right, as Sean already hinted. However, even this limited backport provides a big performance fix for our use case!
As a compromise of sorts, I propose that we disable the TDP MMU by default on v5.15, and backport these fixes to v6.1. v5.15 and earlier won't get "ludicrous speed", but I think that's perfectly acceptable since KVM has had the suboptimal behavior literally since EPT/NPT support was first added.
Disabling TDP MMU for v5.15, and backporting things to v6.1 works for me.
I'm comfortable backporting to v6.1 as that is recent enough, and there weren't substantial MMU changes between v6.1 and v6.3 in this area. I.e. I have a decent level of confidence that we aren't overlooking some subtle dependency.
For v5.15, I am less confident in the safety of a backport, and more importantly, I think we should disable the TDP MMU by default to mitigate the underlying flaw that makes the 18x speedup possible. That flaw is that KVM can end up freeing and rebuilding TDP MMU roots every time CR0.WP is toggled or a vCPU transitions to/from SMM.
The interesting thing here is that these CR0.WP fixes seem to improve things with legacy MMU as well, and legacy MMU is not affected/touched by [3].
Yep, that's totally expected. The final patch in this series allows KVM to elide VM-Exits when the guest toggles CR0.WP (but only on Intel hardware). Avoiding VM-Exit entirely is a big performance win when the guest is constantly toggling CR0.WP, e.g. each exit is roughly 1500 cycles, versus probalby something like ~50 for a native write to CR0.WP.
So I think you can consider Mathias' ask independent of disabling TDP MMU. On the one hand: there is no regression here. On the other: the gain is big and seems important to him.
Ya, that's the compromise I am proposing. Give v6.1 the full tune-up, but only do the super safe change for v5.15.
On 4/14/2023 10:17 PM, Sean Christopherson wrote:
On Fri, Apr 14, 2023, Jeremi Piotrowski wrote:
On Fri, Apr 14, 2023 at 09:49:28AM -0700, Sean Christopherson wrote:
+Jeremi
Adding myself :)
/facepalm
This isn't some mundane detail, Michael!!!
On Fri, Apr 14, 2023, Mathias Krause wrote:
...
OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU on v5.10.
Anyone that's enabling the TDP MMU on v5.10 is on their own, we didn't enable the TDP MMU by default until v5.14 for very good reasons.
I backported the whole series down to v5.10 but left out the CR0.WP guest owning patch+fix for v5.4 as the code base is too different to get all the nuances right, as Sean already hinted. However, even this limited backport provides a big performance fix for our use case!
As a compromise of sorts, I propose that we disable the TDP MMU by default on v5.15, and backport these fixes to v6.1. v5.15 and earlier won't get "ludicrous speed", but I think that's perfectly acceptable since KVM has had the suboptimal behavior literally since EPT/NPT support was first added.
Disabling TDP MMU for v5.15, and backporting things to v6.1 works for me.
I'm comfortable backporting to v6.1 as that is recent enough, and there weren't substantial MMU changes between v6.1 and v6.3 in this area. I.e. I have a decent level of confidence that we aren't overlooking some subtle dependency.
For v5.15, I am less confident in the safety of a backport, and more importantly, I think we should disable the TDP MMU by default to mitigate the underlying flaw that makes the 18x speedup possible. That flaw is that KVM can end up freeing and rebuilding TDP MMU roots every time CR0.WP is toggled or a vCPU transitions to/from SMM.
So given that there hasn't been any further comms, I assume we stick to the plan outlined above: disable tdp_mmu by default for 5.15.
Should that just be a revert of 71ba3f3189c78f756a659568fb473600fd78f207 ("KVM: x86: enable TDP MMU by default") or a new patch and more importantly - do you want to post the patch Sean, or are you busy and would prefer if someone else did?
Jeremi
Sorry for the late reply, I've been traveling the past three weeks.
On 14.04.23 18:49, Sean Christopherson wrote:
+Jeremi
On Fri, Apr 14, 2023, Mathias Krause wrote:
On 06.04.23 15:22, Mathias Krause wrote:
On 06.04.23 04:25, Sean Christopherson wrote:
These are quite risky to backport. E.g. we botched patch 6[*], and my initial fix also had a subtle bug. There have also been quite a few KVM MMU changes since 5.4, so it's possible that an edge case may exist in 5.4 that doesn't exist in mainline.
I totally agree. Getting the changes to work with older kernels needs more work. The MMU role handling was refactored in 5.14 and down to 5.4 it differs even more, so backports to earlier kernels definitely needs more care.
My plan would be to limit backporting of the whole series to kernels down to 5.15 (maybe 5.10 if it turns out to be doable) and for kernels before that only without patch 6. That would leave out the problematic change but still give us the benefits of dropping the needless mmu unloads for only toggling CR0.WP in the VM. This already helps us a lot!
To back up the "helps us a lot" with some numbers, here are the results I got from running the 'ssdd 10 50000' micro-benchmark on the backports I did, running on a grsecurity L1 VM (host is a vanilla kernel, as stated below; runtime in seconds, lower is better):
legacy TDP shadow Linux v5.4.240 - 8.87s 56.8s + patches - 5.84s 55.4sI believe "legacy" and "TDP" are flip-flopped, the TDP MMU does't exist in v5.4.
Well, whatever the meaning of "TDP" is in v5.4 -- 'tdp_enabled' completely mirrors the value of 'enable_ept' / 'npt_enabled'. But yeah, it probably means what "legacy" is for newer kernels.
Linux v5.10.177 10.37s 88.7s 69.7s + patches 4.88s 4.92s 70.1s Linux v5.15.106 9.94s 66.1s 64.9s + patches 4.81s 4.79s 64.6s Linux v6.1.23 7.65s 8.23s 68.7s + patches 3.36s 3.36s 69.1s Linux v6.2.10 7.61s 7.98s 68.6s + patches 3.37s 3.41s 70.2sI guess we can grossly ignore the shadow MMU numbers, beside noting them to regress from v5.4 to v5.10 (something to investigate?). The backports don't help (much) for shadow MMU setups and the flux in the measurements is likely related to the slab allocations involved.
Another unrelated data point is that TDP MMU is really broken for our use case on v5.10 and v5.15 -- it's even slower that shadow paging!
OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU on v5.10.
Anyone that's enabling the TDP MMU on v5.10 is on their own, we didn't enable the TDP MMU by default until v5.14 for very good reasons.
Fair enough. But the numbers don't look much better for v5.15, so we still want to fix that performance degradation when using TDP MMU (we used to have a patch that disables TDP MMU in grsec by default but this, of course, has no impact on setups making use of a vanilla / distro host kernel and using grsec in the guest VM).
I backported the whole series down to v5.10 but left out the CR0.WP guest owning patch+fix for v5.4 as the code base is too different to get all the nuances right, as Sean already hinted. However, even this limited backport provides a big performance fix for our use case!
As a compromise of sorts, I propose that we disable the TDP MMU by default on v5.15, and backport these fixes to v6.1. v5.15 and earlier won't get "ludicrous speed", but I think that's perfectly acceptable since KVM has had the suboptimal behavior literally since EPT/NPT support was first added.
The issue only started to get really bad when TDP MMU was enabled by default in 5.14. That's why we reverted that change in grsecurity right away. Integrating that change upstream will get us back to the pre-5.14 performance numbers but why not do better and fix the underlying bug by backporting this series?
I'm comfortable backporting to v6.1 as that is recent enough, and there weren't substantial MMU changes between v6.1 and v6.3 in this area. I.e. I have a decent level of confidence that we aren't overlooking some subtle dependency.
Agreed, the backports down to v6.1 were trivial.
For v5.15, I am less confident in the safety of a backport, and more importantly, I think we should disable the TDP MMU by default to mitigate the underlying flaw that makes the 18x speedup possible. That flaw is that KVM can end up freeing and rebuilding TDP MMU roots every time CR0.WP is toggled or a vCPU transitions to/from SMM.
For v5.15 a few more commits are needed to ensure all requirements are met, like no guest owned CR4 bits overlap with KVM's MMU role. But that's manageable, IMHO, as some parts already went into previous stable updates, making the missing net diff for the backport +6 -5 lines.
We mitigated the CR0.WP case between v5.15 and v6.1[1], which is why v6.1 doesn't exhibit the same pain as v5.10, but Jeremi discovered that the SMM case badly affects KVM-on-HyperV[2], e.g. when lauching KVM guests using WSL. I posted a fix[3] to finally resolve the underlying bug, but as Jeremi discovered[4], backporting the fix to v5.15 is going to be gnarly, to say the least. It'll be far worse than backporting these CR0.WP patches, and maybe even infeasible without a large scale rework (no thanks).
So disabling TDP MMU for v5.15 seems to mitigate both performance degradation, Jeremi's and ours. However, I'd like to get the extra speedup still. As I wrote, I already did the backports and it wasn't all that bad in the end. I just had to read and map the code of older kernels to what newer kernels would do and it's not that far away, actually. It's the cleanup patches that just make it look a lot a different, but for the MMU role it's actually not all that different. It's completely different story for v5.4, sure. But I don't propose to backport the full series to that kernel either.
Anyone that will realize meaningful benefits from the TDP MMU is all but guaranteed to be rolling their own kernels, i.e. can do the backports themselves if they want to use a v5.15 based kernel. The big selling point of the TDP MMU is that it scales better to hundreds of vCPUs, particularly when live migrating such VMs. I highly doubt that anyone running a stock kernel is running 100+ vCPU VMs, let alone trying to live migrate them.
I'll post the backports I did and maybe can convince you as well that it's not all that bad ;) But I see your proposal patch [3] got merged in the meantime and is cc:stable. Might make sense to re-do my benchmarks after it got applied to the older kernels.
Thanks, Mathias
[1] https://lkml.kernel.org/r/20220209170020.1775368-1-pbonzini%40redhat.com [2] https://lore.kernel.org/all/959c5bce-beb5-b463-7158-33fc4a4f910c@linux.micro... [3] https://lore.kernel.org/all/20230413231251.1481410-1-seanjc@google.com [4] https://lore.kernel.org/all/7332d846-fada-eb5c-6068-18ff267bd37f@linux.micro...
On 08.05.23 11:19, Mathias Krause wrote:
[...]
I'll post the backports I did and maybe can convince you as well that it's not all that bad ;) But I see your proposal patch [3] got merged in the meantime and is cc:stable. Might make sense to re-do my benchmarks after it got applied to the older kernels.
I just sent out the backports to the mailing list, please have a look!
The benchmark numbers are still the old ones I did three weeks ago. However, seeing what's needed for the backport might give you a better feeling for the impact.
I'll refresh the series if there's demand and when edbdb43fc96b ("KVM: x86: Preserve TDP MMU roots until they are explicitly invalidated") got merged into the relevant kernels.
v6.2: https://lore.kernel.org/stable/20230508154457.29956-1-minipli@grsecurity.net... v6.1: https://lore.kernel.org/stable/20230508154602.30008-1-minipli@grsecurity.net... v5.15: https://lore.kernel.org/stable/20230508154709.30043-1-minipli@grsecurity.net... v5.10: https://lore.kernel.org/stable/20230508154804.30078-1-minipli@grsecurity.net... v5.4: https://lore.kernel.org/stable/20230508154943.30113-1-minipli@grsecurity.net...
Thanks, Mathias
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