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

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Date:   Sat, 7 Oct 2023 15:00:41 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Yan Zhao <yan.y.zhao@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     pbonzini@...hat.com, chao.gao@...el.com, kai.huang@...el.com,
        robert.hoo.linux@...il.com, yuan.yao@...ux.intel.com,
        kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 01/12] KVM: x86/mmu: helpers to return if KVM honors
 guest MTRRs

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@...gle.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.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)
>   {
>   	/*