KVM: x86: remove shadow_memtype_mask

JIRA: https://issues.redhat.com/browse/RHEL-47242

commit 3fee483
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Mon Mar 3 11:31:50 2025 -0500

    KVM: x86: remove shadow_memtype_mask

    The IGNORE_GUEST_PAT quirk is inapplicable, and thus always-disabled,
    if shadow_memtype_mask is zero.  As long as vmx_get_mt_mask is not
    called for the shadow paging case, there is no need to consult
    shadow_memtype_mask and it can be removed altogether.

    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Author: Maxim Levitsky

arch/x86/kvm/mmu/mmu.c

@@ -4836,21 +4836,6 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
 }
 #endif
 
-bool kvm_mmu_may_ignore_guest_pat(struct kvm *kvm)
-{
-	/*
-	 * 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 guest PAT, when EPT is enabled and when quirk
-	 * KVM_X86_QUIRK_IGNORE_GUEST_PAT is enabled or the CPU lacks the
-	 * ability to safely honor guest PAT.
-	 */
-	return kvm_check_has_quirk(kvm, KVM_X86_QUIRK_IGNORE_GUEST_PAT);
-}
-
 int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 #ifdef CONFIG_X86_64

arch/x86/kvm/mmu/spte.c

@@ -37,7 +37,6 @@ u64 __read_mostly shadow_mmio_value;
 u64 __read_mostly shadow_mmio_mask;
 u64 __read_mostly shadow_mmio_access_mask;
 u64 __read_mostly shadow_present_mask;
-u64 __read_mostly shadow_memtype_mask;
 u64 __read_mostly shadow_me_value;
 u64 __read_mostly shadow_me_mask;
 u64 __read_mostly shadow_acc_track_mask;
@@ -210,9 +209,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	if (level > PG_LEVEL_4K)
 		spte |= PT_PAGE_SIZE_MASK;
 
-	if (shadow_memtype_mask)
-		spte |= kvm_x86_call(get_mt_mask)(vcpu, gfn,
-						  kvm_is_mmio_pfn(pfn));
+	spte |= kvm_x86_call(get_mt_mask)(vcpu, gfn, kvm_is_mmio_pfn(pfn));
 	if (host_writable)
 		spte |= shadow_host_writable_mask;
 	else
@@ -467,13 +464,7 @@ void kvm_mmu_set_ept_masks(bool has_ad_bits, bool has_exec_only)
 	/* VMX_EPT_SUPPRESS_VE_BIT is needed for W or X violation. */
 	shadow_present_mask	=
 		(has_exec_only ? 0ull : VMX_EPT_READABLE_MASK) | VMX_EPT_SUPPRESS_VE_BIT;
-	/*
-	 * EPT overrides the host MTRRs, and so KVM must program the desired
-	 * memtype directly into the SPTEs.  Note, this mask is just the mask
-	 * of all bits that factor into the memtype, the actual memtype must be
-	 * dynamically calculated, e.g. to ensure host MMIO is mapped UC.
-	 */
-	shadow_memtype_mask	= VMX_EPT_MT_MASK | VMX_EPT_IPAT_BIT;
+
 	shadow_acc_track_mask	= VMX_EPT_RWX_MASK;
 	shadow_host_writable_mask = EPT_SPTE_HOST_WRITABLE;
 	shadow_mmu_writable_mask  = EPT_SPTE_MMU_WRITABLE;
@@ -525,12 +516,6 @@ void kvm_mmu_reset_all_pte_masks(void)
 	shadow_x_mask		= 0;
 	shadow_present_mask	= PT_PRESENT_MASK;
 
-	/*
-	 * For shadow paging and NPT, KVM uses PAT entry '0' to encode WB
-	 * memtype in the SPTEs, i.e. relies on host MTRRs to provide the
-	 * correct memtype (WB is the "weakest" memtype).
-	 */
-	shadow_memtype_mask	= 0;
 	shadow_acc_track_mask	= 0;
 	shadow_me_mask		= 0;
 	shadow_me_value		= 0;

arch/x86/kvm/mmu/spte.h

@@ -187,7 +187,6 @@ extern u64 __read_mostly shadow_mmio_value;
 extern u64 __read_mostly shadow_mmio_mask;
 extern u64 __read_mostly shadow_mmio_access_mask;
 extern u64 __read_mostly shadow_present_mask;
-extern u64 __read_mostly shadow_memtype_mask;
 extern u64 __read_mostly shadow_me_value;
 extern u64 __read_mostly shadow_me_mask;
 

arch/x86/kvm/vmx/vmx.c

@@ -8527,6 +8527,8 @@ __init int vmx_hardware_setup(void)
 	if (enable_ept)
 		kvm_mmu_set_ept_masks(enable_ept_ad_bits,
 				      cpu_has_vmx_ept_execute_only());
+	else
+		vt_x86_ops.get_mt_mask = NULL;
 
 	/*
 	 * Setup shadow_me_value/shadow_me_mask to include MKTME KeyID

arch/x86/kvm/x86.c

@@ -13566,8 +13566,10 @@ static void kvm_noncoherent_dma_assignment_start_or_stop(struct kvm *kvm)
 	 * due to toggling the "ignore PAT" bit.  Zap all SPTEs when the first
 	 * (or last) non-coherent device is (un)registered to so that new SPTEs
 	 * with the correct "ignore guest PAT" setting are created.
+	 *
+	 * If KVM always honors guest PAT, however, there is nothing to do.
 	 */
-	if (kvm_mmu_may_ignore_guest_pat(kvm))
+	if (kvm_check_has_quirk(kvm, KVM_X86_QUIRK_IGNORE_GUEST_PAT))
 		kvm_zap_gfn_range(kvm, gpa_to_gfn(0), gpa_to_gfn(~0ULL));
 }