Re: [PATCH v2 1/3] KVM: x86: Track supported ARCH_CAPABILITIES in kvm_caps

[Xiaoyao Li <xiaoyao.li@xxxxxxxxx>]

On 5/24/2023 2:16 PM, Chao Gao wrote:
> to avoid computing the supported value at runtime every time.
>
> Toggle the ARCH_CAP_SKIP_VMENTRY_L1DFLUSH bit when l1tf_vmx_mitigation
> is modified to achieve the same result as runtime computing.

It's not the same result.

In kvm_get_arch_capabilities(), host's value is honored. I.e., when host 
supports ARCH_CAP_SKIP_VMENTRY_L1DFLUSH, l1tf_vmx_mitigation doesn't 
make any difference to the result.

> Opportunistically, add a comment to document the problem of allowing
> changing the supported value of ARCH_CAPABILITIES and the reason why
> we don't fix it.
>
> No functional change intended.
>
> Link: https://lore.kernel.org/all/ZGZhW%2Fx5OWPmx1qD@xxxxxxxxxx/
> Link: https://lore.kernel.org/all/ZGeU9sYTPxqNGSqI@xxxxxxxxxx/
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> ---
>   arch/x86/kvm/vmx/vmx.c | 25 +++++++++++++++++++++++--
>   arch/x86/kvm/x86.c     |  7 ++++---
>   arch/x86/kvm/x86.h     |  1 +
>   3 files changed, 28 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 44fb619803b8..8274ef5e89e5 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -309,10 +309,31 @@ static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
>   
>   	l1tf_vmx_mitigation = l1tf;
>   
> -	if (l1tf != VMENTER_L1D_FLUSH_NEVER)
> +	/*
> +	 * Update static keys and supported arch capabilities according to
> +	 * the new mitigation state.
> +	 *
> +	 * ARCH_CAP_SKIP_VMENTRY_L1DFLUSH is toggled because if we do cache
> +	 * flushes for L1 guests on (nested) vmlaunch/vmresume to L2, L1
> +	 * guests can skip the flush and if we don't, then L1 guests need
> +	 * to do a flush.
> +	 *
> +	 * Toggling ARCH_CAP_SKIP_VMENTRY_L1DFLUSH may present inconsistent
> +	 * model to the guest, e.g., if userspace isn't careful, a VM can
> +	 * have vCPUs with different values for ARCH_CAPABILITIES. But
> +	 * there is almost no chance to fix the issue. Because, to present
> +	 * a consistent model, KVM essentially needs to disallow changing
> +	 * the module param after VMs/vCPUs have been created, but that
> +	 * would prevent userspace from toggling the param while VMs are
> +	 * running, e.g., in response to a new vulnerability.
> +	 */
> +	if (l1tf != VMENTER_L1D_FLUSH_NEVER) {
>   		static_branch_enable(&vmx_l1d_should_flush);
> -	else
> +		kvm_caps.supported_arch_cap |= ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
> +	} else {
>   		static_branch_disable(&vmx_l1d_should_flush);
> +		kvm_caps.supported_arch_cap &= ~ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
> +	}
>   
>   	if (l1tf == VMENTER_L1D_FLUSH_COND)
>   		static_branch_enable(&vmx_l1d_flush_cond);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c0778ca39650..2408b5f554b7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1672,7 +1672,7 @@ 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;
> @@ -7156,7 +7156,7 @@ static void kvm_probe_msr_to_save(u32 msr_index)
>   			return;
>   		break;
>   	case MSR_IA32_TSX_CTRL:
> -		if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
> +		if (!(kvm_caps.supported_arch_cap & ARCH_CAP_TSX_CTRL_MSR))
>   			return;
>   		break;
>   	default:
> @@ -9532,6 +9532,7 @@ 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;
>   
> @@ -11895,7 +11896,7 @@ 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);