Re: [PATCH v15 07/41] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

Binbin Wu
2025-09-16T07:23:45Z

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>
>
> Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
> due to XSS MSR modification.
> CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
> xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
> before allocate sufficient xsave buffer.
>
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
>
> Opportunistically skip CPUID updates if XSS value doesn't change.
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> 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>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Reviewed-by: Chao Gao <chao.gao@intel.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
> arch/x86/kvm/cpuid.c | 3 ++-
> arch/x86/kvm/x86.c | 2 ++
> 2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 46cf616663e6..b5f87254ced7 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -316,7 +316,8 @@ static void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> best = kvm_find_cpuid_entry_index(vcpu, 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);
> + best->ebx = xstate_required_size(vcpu->arch.xcr0 |
> + vcpu->arch.ia32_xss, true);
> }
>
> static bool kvm_cpuid_has_hyperv(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5a5af40c06a9..519d58b82f7f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3993,6 +3993,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> */
> if (data & ~vcpu->arch.guest_supported_xss)
> return 1;
> + if (vcpu->arch.ia32_xss == data)
> + break;
> vcpu->arch.ia32_xss = data;
> vcpu->arch.cpuid_dynamic_bits_dirty = true;
> break;