Re: [PATCH v2 0/4] KVM: x86: Introduce quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT

[Yan Zhao]

On Sat, Mar 01, 2025 at 02:34:24AM -0500, Paolo Bonzini wrote:
> This series is my evolution of Yan's patches at
> https://patchew.org/linux/20250224070716.31360-1-yan.y.zhao@xxxxxxxxx/.
Hi Paolo,
Thanks for helping refining the patches!

Here's a summary of my comments:
1. This is confusing for KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT to be present
   on AMD's platforms while not present on Intel's non-self-snoop platforms.
   (patch 2)

2. Could we make KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT and
   KVM_X86_QUIRK_SLOT_ZAP_ALL always-disabled for TDs? (patch 4)

3. kvm_caps.inapplicable_quirks may not be necessary. (patches 1/3)

Thanks
Yan
> 
> The implementation of the quirk is unchanged, but the concepts in kvm_caps
> are a bit different.  In particular:
> 
> - if a quirk is not applicable to some hardware, it is still included
>   in KVM_CAP_DISABLE_QUIRKS2.  This way userspace knows that KVM is
>   *aware* of a particular issue - even if disabling it has no effect
>   because the quirk is not a problem on a specific hardware, userspace
>   may want to know that it can rely on the problematic behavior not
>   being present.  Therefore, KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT is
>   simply auto-disabled on TDX machines.
>
> - if instead a quirk cannot be disabled due to limitations, for example
>   KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT if self-snoop is not present on
>   the CPU, the quirk is removed completely from kvm_caps.supported_quirks
>   and therefore from KVM_CAP_DISABLE_QUIRKS2.
> 
> This series does not introduce a way to query always-disabled quirks,
> which could be for example KVM_CAP_DISABLED_QUIRKS.  This could be
> added if we wanted for example to get rid of hypercall patching; it's
> a trivial addition.

> 
> Paolo Bonzini (1):
>   KVM: x86: Allow vendor code to disable quirks
> 
> Yan Zhao (3):
>   KVM: x86: Introduce supported_quirks to block disabling quirks
>   KVM: x86: Introduce Intel specific quirk
>     KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT
>   KVM: TDX: Always honor guest PAT on TDX enabled platforms
> 
>  Documentation/virt/kvm/api.rst  | 22 ++++++++++++++++++
>  arch/x86/include/uapi/asm/kvm.h |  1 +
>  arch/x86/kvm/mmu.h              |  2 +-
>  arch/x86/kvm/mmu/mmu.c          | 11 +++++----
>  arch/x86/kvm/svm/svm.c          |  1 +
>  arch/x86/kvm/vmx/tdx.c          |  6 +++++
>  arch/x86/kvm/vmx/vmx.c          | 40 +++++++++++++++++++++++++++------
>  arch/x86/kvm/x86.c              | 10 +++++----
>  arch/x86/kvm/x86.h              | 14 +++++++-----
>  9 files changed, 86 insertions(+), 21 deletions(-)
> 
> -- 
> 2.43.5
>