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KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

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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>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Yang Weijiang 2025-09-19 15:32:14 -07:00 committed by Sean Christopherson
parent 338543cbe0
commit 9622e116d0
2 changed files with 4 additions and 1 deletions
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3
arch/x86/kvm/cpuid.c
View File

@ -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)

2
arch/x86/kvm/x86.c
View File

@ -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;