On 8/21/2025 9:30 PM, Chao Gao 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 return KVM_MSR_RET_UNSUPPORTED if guest CPUID doesn't
> enumerate it. Since KVM_MSR_RET_UNSUPPORTED takes care of host_initiated
> cases, drop the host_initiated check.
It looks most of this patch is introducing
vcpu->arch.guest_supported_xss
and use it to guard the wrmsr of MSR_IA32_XSS instead of KVM global
kvm_caps.supported_xss.
So it's better to split it into two patches?
> 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>
> 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>
> ---
> arch/x86/include/asm/kvm_host.h | 3 ++-
> arch/x86/kvm/cpuid.c | 15 ++++++++++++++-
> arch/x86/kvm/x86.c | 9 +++++----
> 3 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0d3cc0fc27af..b7f82a421718 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -815,7 +815,6 @@ struct kvm_vcpu_arch {
> bool at_instruction_boundary;
> bool tpr_access_reporting;
> bool xfd_no_write_intercept;
> - u64 ia32_xss;
> u64 microcode_version;
> u64 arch_capabilities;
> u64 perf_capabilities;
> @@ -876,6 +875,8 @@ struct kvm_vcpu_arch {
>
> u64 xcr0;
> u64 guest_supported_xcr0;
> + u64 guest_supported_xss;
> + u64 ia32_xss;
>
> struct kvm_pio_request pio;
> void *pio_data;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index ad6cadf09930..b5f87254ced7 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -263,6 +263,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_vcpu *vcpu)
> return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> }
>
> +static u64 cpuid_get_supported_xss(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_cpuid_entry2 *best;
> +
> + best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
> + if (!best)
> + return 0;
> +
> + return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
> +}
> +
> static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
> struct kvm_cpuid_entry2 *entry,
> unsigned int x86_feature,
> @@ -305,7 +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)
> @@ -424,6 +436,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> }
>
> vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu);
> + vcpu->arch.guest_supported_xss = cpuid_get_supported_xss(vcpu);
>
> vcpu->arch.pv_cpuid.features = kvm_apply_cpuid_pv_features_quirk(vcpu);
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 569583943779..75b7a29721bb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4011,16 +4011,17 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> }
> break;
> case MSR_IA32_XSS:
> - if (!msr_info->host_initiated &&
> - !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> - return 1;
> + if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> + return KVM_MSR_RET_UNSUPPORTED;
> /*
> * KVM supports exposing PT to the guest, but does not support
> * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
> * XSAVES/XRSTORS to save/restore PT MSRs.
> */
> - if (data & ~kvm_caps.supported_xss)
> + 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;