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KVM: x86: Introduce Intel specific quirk KVM_X86_QUIRK_IGNORE_GUEST_PAT

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Introduce an Intel specific quirk KVM_X86_QUIRK_IGNORE_GUEST_PAT to have
KVM ignore guest PAT when this quirk is enabled.

On AMD platforms, KVM always honors guest PAT.  On Intel however there are
two issues.  First, KVM *cannot* honor guest PAT if CPU feature self-snoop
is not supported. Second, UC access on certain Intel platforms can be very
slow[1] and honoring guest PAT on those platforms may break some old
guests that accidentally specify video RAM as UC. Those old guests may
never expect the slowness since KVM always forces WB previously. See [2].

So, introduce a quirk that KVM can enable by default on all Intel platforms
to avoid breaking old unmodifiable guests. Newer userspace can disable this
quirk if it wishes KVM to honor guest PAT; disabling the quirk will fail
if self-snoop is not supported, i.e. if KVM cannot obey the wish.

The quirk is a no-op on AMD and also if any assigned devices have
non-coherent DMA.  This is not an issue, as KVM_X86_QUIRK_CD_NW_CLEARED is
another example of a quirk that is sometimes automatically disabled.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/all/Ztl9NWCOupNfVaCA@yzhao56-desk.sh.intel.com # [1]
Link: https://lore.kernel.org/all/87jzfutmfc.fsf@redhat.com # [2]
Message-ID: <20250224070946.31482-1-yan.y.zhao@intel.com>
[Use supported_quirks/inapplicable_quirks to support both AMD and
 no-self-snoop cases, as well as to remove the shadow_memtype_mask check
 from kvm_mmu_may_ignore_guest_pat(). - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Yan Zhao
2025-02-24 15:09:45 +08:00
committed by Paolo Bonzini
parent bd7d5362b4
commit c9c1e20b4c
7 changed files with 73 additions and 15 deletions
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22
Documentation/virt/kvm/api.rst
View File

@@ -8160,6 +8160,28 @@ KVM_X86_QUIRK_STUFF_FEATURE_MSRS By default, at vCPU creation, KVM sets the
and 0x489), as KVM does now allow them to
be set by userspace (KVM sets them based on
guest CPUID, for safety purposes).
KVM_X86_QUIRK_IGNORE_GUEST_PAT By default, on Intel platforms, KVM ignores
guest PAT and forces the effective memory
type to WB in EPT. The quirk is not available
on Intel platforms which are incapable of
safely honoring guest PAT (i.e., without CPU
self-snoop, KVM always ignores guest PAT and
forces effective memory type to WB). It is
also ignored on AMD platforms or, on Intel,
when a VM has non-coherent DMA devices
assigned; KVM always honors guest PAT in
such case. The quirk is needed to avoid
slowdowns on certain Intel Xeon platforms
(e.g. ICX, SPR) where self-snoop feature is
supported but UC is slow enough to cause
issues with some older guests that use
UC instead of WC to map the video RAM.
Userspace can disable the quirk to honor
guest PAT if it knows that there is no such
guest software, for example if it does not
expose a bochs graphics device (which is
known to have had a buggy driver).
=================================== ============================================
7.32 KVM_CAP_MAX_VCPU_ID