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Do not always honor guest PAT on CPUs that support self-snoop.

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This triggers an issue in the bochsdrm driver, which used ioremap()
 instead of ioremap_wc() to map the video RAM.  The revert lets video
 RAM use the WB memory type instead of the slower UC memory type.
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Merge tag 'for-linus-6.11' of git://git.kernel.org/pub/scm/virt/kvm/kvm

Pull kvm fix from Paolo Bonzini:
 "Do not always honor guest PAT on CPUs that support self-snoop.

  This triggers an issue in the bochsdrm driver, which used ioremap()
  instead of ioremap_wc() to map the video RAM.

  The revert lets video RAM use the WB memory type instead of the slower
  UC memory type"

* tag 'for-linus-6.11' of git://git.kernel.org/pub/scm/virt/kvm/kvm:
  Revert "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop"
This commit is contained in:
Linus Torvalds 2024-09-15 09:35:50 +02:00
commit d42f7708e2
2 changed files with 7 additions and 11 deletions
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8
arch/x86/kvm/mmu/mmu.c
View File

@ -4674,16 +4674,14 @@ out_unlock:
bool kvm_mmu_may_ignore_guest_pat(void)
{
/*
* When EPT is enabled (shadow_memtype_mask is non-zero), the CPU does
* not support self-snoop (or is affected by an erratum), and the VM
* When EPT is enabled (shadow_memtype_mask is non-zero), and the VM
* has non-coherent DMA (DMA doesn't snoop CPU caches), KVM's ABI is to
* honor the memtype from the guest's PAT so that guest accesses to
* memory that is DMA'd aren't cached against the guest's wishes. As a
* result, KVM _may_ ignore guest PAT, whereas without non-coherent DMA,
* KVM _always_ ignores or honors guest PAT, i.e. doesn't toggle SPTE
* bits in response to non-coherent device (un)registration.
* KVM _always_ ignores guest PAT (when EPT is enabled).
*/
return !static_cpu_has(X86_FEATURE_SELFSNOOP) && shadow_memtype_mask;
return shadow_memtype_mask;
}
int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)

10
arch/x86/kvm/vmx/vmx.c
View File

@ -7659,13 +7659,11 @@ u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
/*
* Force WB and ignore guest PAT if the VM does NOT have a non-coherent
* device attached and the CPU doesn't support self-snoop. Letting the
* guest control memory types on Intel CPUs without self-snoop may
* result in unexpected behavior, and so KVM's (historical) ABI is to
* trust the guest to behave only as a last resort.
* device attached. Letting the guest control memory types on Intel
* CPUs may result in unexpected behavior, and so KVM's ABI is to trust
* the guest to behave only as a last resort.
*/
if (!static_cpu_has(X86_FEATURE_SELFSNOOP) &&
!kvm_arch_has_noncoherent_dma(vcpu->kvm))
if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;
return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT);