Commit 9622e116 authored Sep 19, 2025 by Yang Weijiang Committed by Sean Christopherson Sep 23, 2025

KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS



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>

parent 338543cb

arch/x86/kvm/cpuid.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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)

arch/x86/kvm/x86.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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;