Re: [PATCH v2 03/15] KVM: x86: Refresh CPUID on writes to MSR_IA32_XSS

Sean Christopherson
2023-01-26T19:50:48Z

On Thu, Nov 24, 2022, Yang Weijiang wrote:
> Updated CPUID.0xD.0x1, which reports the current required storage size
> of all features enabled via XCR0 | XSS, when the guest's XSS is modified.
>
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> ---
> arch/x86/kvm/cpuid.c | 16 +++++++++++++---
> arch/x86/kvm/x86.c | 6 ++++--
> 2 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6b5912578edd..85e3df6217af 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -272,9 +272,19 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
> best->ebx = xstate_required_size(vcpu->arch.xcr0, false);
>
> best = cpuid_entry2_find(entries, nent, 0xD, 1);
> - if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
> - cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> - best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> + if (best) {
> + if (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
> + cpuid_entry_has(best, X86_FEATURE_XSAVEC)) {
> + u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
> +
> + best->ebx = xstate_required_size(xstate, true);
> + }
> +
> + if (!cpuid_entry_has(best, X86_FEATURE_XSAVES)) {
> + best->ecx = 0;
> + best->edx = 0;

ECX and EDX should be left alone, it is userspace's responsibility to provide a
sane CPUID model. E.g. KVM doesn't clear EBX or EDX in CPUID.0xD.0x1 when XSAVE
is unsupported.

> + }
> + }
>
> best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
> if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 16726b44061b..888a153e32bc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3685,8 +3685,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> */
> if (data & ~kvm_caps.supported_xss)
> return 1;
> - vcpu->arch.ia32_xss = data;
> - kvm_update_cpuid_runtime(vcpu);
> + if (vcpu->arch.ia32_xss != data) {
> + vcpu->arch.ia32_xss = data;
> + kvm_update_cpuid_runtime(vcpu);
> + }
> break;
> case MSR_SMI_COUNT:
> if (!msr_info->host_initiated)
> --
> 2.27.0
>