Commit ffe9d796 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

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.

Tested-by: default avatarPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/r/20250523011756.3243624-3-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent c126b46e

arch/x86/kvm/mmu/spte.c

+18 −4
Original line number Diff line number Diff line
@@ -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;