linux-kernel - Re: [PATCH] KVM: x86: Track supported ARCH_CAPABILITIES in kvm

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Date:   Thu, 18 May 2023 17:32:31 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Track supported ARCH_CAPABILITIES in kvm_caps

On 5/6/2023 11:04 AM, Chao Gao wrote:
> to avoid computing the supported value at runtime every time.
> 
> No functional change intended.

the value of kvm_get_arch_capabilities() can be changed due to

	if (l1tf_vmx_mitigation != VMENTER_L1D_FLUSH_NEVER)
		data |= ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;

and l1tf_vmx_mitigation can be runtime changed by vmentry_l1d_flush 
module param.

We need a detailed analysis that in no real case can 
ARCH_CAP_SKIP_VMENTRY_L1DFLUSH bit change runtime.

> Signed-off-by: Chao Gao <chao.gao@...el.com>
> ---
> 
> A new call site of kvm_get_arch_capabilities() is added by [1]. It should be
> replaced with the cached value in kvm_caps.
> 
> [1] https://lore.kernel.org/all/20230504181827.130532-1-mizhang@google.com/
> 
> 
>   arch/x86/kvm/x86.c | 5 +++--
>   arch/x86/kvm/x86.h | 1 +
>   2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 523c39a03c00..94aa70ec169c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1670,7 +1670,7 @@ static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
>   {
>   	switch (msr->index) {
>   	case MSR_IA32_ARCH_CAPABILITIES:
> -		msr->data = kvm_get_arch_capabilities();
> +		msr->data = kvm_caps.supported_arch_cap;
>   		break;
>   	case MSR_IA32_PERF_CAPABILITIES:
>   		msr->data = kvm_caps.supported_perf_cap;
> @@ -9523,6 +9523,7 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>   		kvm_caps.max_guest_tsc_khz = max;
>   	}
>   	kvm_caps.default_tsc_scaling_ratio = 1ULL << kvm_caps.tsc_scaling_ratio_frac_bits;
> +	kvm_caps.supported_arch_cap = kvm_get_arch_capabilities();
>   	kvm_init_msr_lists();
>   	return 0;
>   
> @@ -11879,7 +11880,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>   	if (r)
>   		goto free_guest_fpu;
>   
> -	vcpu->arch.arch_capabilities = kvm_get_arch_capabilities();
> +	vcpu->arch.arch_capabilities = kvm_caps.supported_arch_cap;
>   	vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
>   	kvm_xen_init_vcpu(vcpu);
>   	kvm_vcpu_mtrr_init(vcpu);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index c544602d07a3..d3e524bcc169 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -29,6 +29,7 @@ struct kvm_caps {
>   	u64 supported_xcr0;
>   	u64 supported_xss;
>   	u64 supported_perf_cap;
> +	u64 supported_arch_cap;
>   };
>   
>   void kvm_spurious_fault(void);
> 
> base-commit: b3c98052d46948a8d65d2778c7f306ff38366aac