[PATCH v4 01/12] KVM: x86/mmu: helpers to return if KVM honors guest MTRRs

[Yan Zhao]

Added helpers to check if KVM honors guest MTRRs.
The inner helper __kvm_mmu_honors_guest_mtrrs() is also provided to
outside callers for the purpose of checking if guest MTRRs were honored
before stopping non-coherent DMA.

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
 arch/x86/kvm/mmu.h     |  7 +++++++
 arch/x86/kvm/mmu/mmu.c | 15 +++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 92d5a1924fc1..38bd449226f6 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -235,6 +235,13 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
 	return -(u32)fault & errcode;
 }
 
+bool __kvm_mmu_honors_guest_mtrrs(struct kvm *kvm, bool vm_has_noncoherent_dma);
+
+static inline bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm)
+{
+	return __kvm_mmu_honors_guest_mtrrs(kvm, kvm_arch_has_noncoherent_dma(kvm));
+}
+
 void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end);
 
 int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1e5db621241f..b4f89f015c37 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4516,6 +4516,21 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
 }
 #endif
 
+bool __kvm_mmu_honors_guest_mtrrs(struct kvm *kvm, bool vm_has_noncoherent_dma)
+{
+	/*
+	 * If the TDP is enabled, the host MTRRs are ignored by TDP
+	 * (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 MTRRs so that guest accesses to memory that is
+	 * DMA'd aren't cached against the guest's wishes.
+	 *
+	 * Note, KVM may still ultimately ignore guest MTRRs for certain PFNs,
+	 * e.g. KVM will force UC memtype for host MMIO.
+	 */
+	return vm_has_noncoherent_dma && tdp_enabled && shadow_memtype_mask;
+}
+
 int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 	/*
-- 
2.17.1