KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

Unconditionally honor guest PAT on CPUs that support self-snoop, as
Intel has confirmed that CPUs that support self-snoop always snoop caches
and store buffers.  I.e. CPUs with self-snoop maintain cache coherency
even in the presence of aliased memtypes, thus there is no need to trust
the guest behaves and only honor PAT as a last resort, as KVM does today.

Honoring guest PAT is desirable for use cases where the guest has access
to non-coherent DMA _without_ bouncing through VFIO, e.g. when a virtual
(mediated, for all intents and purposes) GPU is exposed to the guest, along
with buffers that are consumed directly by the physical GPU, i.e. which
can't be proxied by the host to ensure writes from the guest are performed
with the correct memory type for the GPU.

Cc: Yiwei Zhang <zzyiwei@google.com>
Suggested-by: Yan Zhao <yan.y.zhao@intel.com>
Suggested-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Link: https://lore.kernel.org/r/20240309010929.1403984-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -4616,14 +4616,16 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
 bool kvm_mmu_may_ignore_guest_pat(void)
 {
 	/*
-	 * When EPT is enabled (shadow_memtype_mask is non-zero), and the VM
+	 * 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
 	 * 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 guest PAT (when EPT is enabled).
+	 * KVM _always_ ignores or honors guest PAT, i.e. doesn't toggle SPTE
+	 * bits in response to non-coherent device (un)registration.
 	 */
-	return shadow_memtype_mask;
+	return !static_cpu_has(X86_FEATURE_SELFSNOOP) && shadow_memtype_mask;
 }
 
 int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)

arch/x86/kvm/vmx/vmx.c

@@ -7667,11 +7667,13 @@ 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.  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.
+	 * 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.
 	 */
-	if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
+	if (!static_cpu_has(X86_FEATURE_SELFSNOOP) &&
+	    !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);