Re: [RFC PATCH 0/2] SEPT SEAMCALL retry proposal

[Sean Christopherson]

On Tue, Dec 17, 2024, Rick P Edgecombe wrote:
> On Thu, 2024-11-21 at 19:51 +0800, Yan Zhao wrote:
> > This SEPT SEAMCALL retry proposal aims to remove patch
> > "[HACK] KVM: TDX: Retry seamcall when TDX_OPERAND_BUSY with operand SEPT"
> > [1] at the tail of v2 series "TDX MMU Part 2".
> 
> We discussed this on the PUCK call. A couple alternatives were considered:
>  - Avoiding 0-step. To handle signals kicking to userspace we could try to add
> code to generate synthetic EPT violations if KVM thinks the 0-step mitigation
> might be active (i.e. the fault was not resolved). The consensus was that this
> would be continuing battle and possibly impossible due normal guest behavior
> triggering the mitigation. 

Specifically, the TDX Module takes its write-lock if the guest takes EPT
violations exits on the same RIP 6 times, i.e. detects forward progress based
purely on the RIP at entry vs. exit.  So a guest that is touching memory in a
loop could trigger zero-step checking even if KVM promptly fixes every EPT
violation.

>  - Pre-faulting all S-EPT, such that contention with AUG won't happen. The
> discussion was that this would only be a temporary solution as the MMU
> operations get more complicated (huge pages, etc). Also there is also
> private/shared conversions and memory hotplug already.
> 
> So we will proceed with this kick+lock+retry solution. The reasoning is to
> optimize for the normal non-contention path, without having an overly
> complicated solution for KVM.
> 
> In all the branch commotion recently, these patches fell out of our dev branch.
> So we just recently integrated then into a 6.13 kvm-coco-queue based branch. We
> need to perform some regression tests based on 6.13 TDP MMU changes. Assuming no
> issues, we can post the 6.13 rebase to included in kvm-coco-queue with
> instructions on which patches to remove from kvm-coco-queue (i.e. the 16
> retries).
> 
> 
> We also briefly touched on the TDX module behavior where guest operations can
> lock NP PTEs. The kick solution doesn't require changing this functionally, but
> it should still be done to help with debugging issues related to KVM's
> contention solution.

And so that KVM developers don't have to deal with customer escalations due to
performance issues caused by known flaws in the TDX module.

> 
> Thanks all for the discussion!