linux-kernel - [PATCH v15 07/41] KVM: x86: Refresh CPUID on write to guest MSR_IA32

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Message-ID: <20250912232319.429659-8-seanjc@google.com>
Date: Fri, 12 Sep 2025 16:22:45 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Mathias Krause <minipli@...ecurity.net>, 
	John Allen <john.allen@....com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Chao Gao <chao.gao@...el.com>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: [PATCH v15 07/41] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

From: Yang Weijiang <weijiang.yang@...el.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@...gle.com>
Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Reviewed-by: Chao Gao <chao.gao@...el.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>
Tested-by: Mathias Krause <minipli@...ecurity.net>
Tested-by: John Allen <john.allen@....com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.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;
-- 
2.51.0.384.g4c02a37b29-goog