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    SubjectRe: [PATCH v4 01/12] KVM: x86/mmu: helpers to return if KVM honors guest MTRRs
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    On 14/7/2023 2:50 pm, Yan Zhao wrote:
    > 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@google.com>
    > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
    > ---
    > 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)

    According to the motivation provided in the comment, the function will no
    longer need to be passed the parameter "struct kvm *kvm" but will rely on
    the global parameters (plus vm_has_noncoherent_dma), removing "*kvm" ?

    > +{
    > + /*
    > + * 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)
    > {
    > /*

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