linux-kernel - [PATCH 2/5] KVM: x86/mmu: Locally cache whether a PFN is host MMIO when making a SPTE

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Message-ID: <20250523011756.3243624-3-seanjc@google.com>
Date: Thu, 22 May 2025 18:17:53 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, 
	Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 2/5] KVM: x86/mmu: Locally cache whether a PFN is host MMIO
 when making a SPTE

When making a SPTE, cache whether or not the target PFN is host MMIO in
order to avoid multiple rounds of the slow path of kvm_is_mmio_pfn(), e.g.
hitting pat_pfn_immune_to_uc_mtrr() in particular can be problematic.  KVM
currently avoids multiple calls by virtue of the two users being mutually
exclusive (.get_mt_mask() is Intel-only, shadow_me_value is AMD-only), but
that won't hold true if/when KVM needs to detect host MMIO mappings for
other reasons, e.g. for mitigating the MMIO Stale Data vulnerability.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/spte.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index f262c380f40e..3f16c91aa042 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -104,7 +104,7 @@ u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access)
 	return spte;
 }
 
-static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
+static bool __kvm_is_mmio_pfn(kvm_pfn_t pfn)
 {
 	if (pfn_valid(pfn))
 		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
@@ -125,6 +125,19 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
 				     E820_TYPE_RAM);
 }
 
+static bool kvm_is_mmio_pfn(kvm_pfn_t pfn, int *is_host_mmio)
+{
+	/*
+	 * Determining if a PFN is host MMIO is relative expensive.  Cache the
+	 * result locally (in the sole caller) to avoid doing the full query
+	 * multiple times when creating a single SPTE.
+	 */
+	if (*is_host_mmio < 0)
+		*is_host_mmio = __kvm_is_mmio_pfn(pfn);
+
+	return *is_host_mmio;
+}
+
 /*
  * Returns true if the SPTE needs to be updated atomically due to having bits
  * that may be changed without holding mmu_lock, and for which KVM must not
@@ -162,6 +175,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 {
 	int level = sp->role.level;
 	u64 spte = SPTE_MMU_PRESENT_MASK;
+	int is_host_mmio = -1;
 	bool wrprot = false;
 
 	/*
@@ -210,14 +224,14 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 		spte |= PT_PAGE_SIZE_MASK;
 
 	if (kvm_x86_ops.get_mt_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, &is_host_mmio));
 	if (host_writable)
 		spte |= shadow_host_writable_mask;
 	else
 		pte_access &= ~ACC_WRITE_MASK;
 
-	if (shadow_me_value && !kvm_is_mmio_pfn(pfn))
+	if (shadow_me_value && !kvm_is_mmio_pfn(pfn, &is_host_mmio))
 		spte |= shadow_me_value;
 
 	spte |= (u64)pfn << PAGE_SHIFT;
-- 
2.49.0.1151.ga128411c76-goog