On 9/9/2025 5:39 PM, Chao Gao wrote:
From: Yang Weijiang <weijiang.yang@xxxxxxxxx>
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.
Nit:
allocate -> allocating.
Otherwise,
Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
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@xxxxxxxxxx>
Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
Tested-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Tested-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
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 6c167117018c..bbae3bf405c7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4020,6 +4020,8 @@ 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;
