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"

2 files changed, 7 insertions, 11 deletions

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index de05a26b0b7d..7813d28b082f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -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)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f18c2d8c7476..733a0c45d1a6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -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);