Messages in this thread | | | Date | Wed, 24 May 2023 16:14:10 +0800 | Subject | Re: [PATCH v2 1/3] KVM: x86: Track supported ARCH_CAPABILITIES in kvm_caps | From | Xiaoyao Li <> |
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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@google.com/ > Link: https://lore.kernel.org/all/ZGeU9sYTPxqNGSqI@google.com/ > Signed-off-by: Chao Gao <chao.gao@intel.com> > --- > 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);
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