linux-kernel - [PATCH v11 07/23] KVM: x86: Refresh CPUID on write to guest MSR_IA32

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Message-ID: <20250704085027.182163-8-chao.gao@intel.com>
Date: Fri,  4 Jul 2025 01:49:38 -0700
From: Chao Gao <chao.gao@...el.com>
To: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	x86@...nel.org,
	seanjc@...gle.com,
	pbonzini@...hat.com,
	dave.hansen@...el.com
Cc: rick.p.edgecombe@...el.com,
	mlevitsk@...hat.com,
	john.allen@....com,
	weijiang.yang@...el.com,
	minipli@...ecurity.net,
	xin@...or.com,
	Zhang Yi Z <yi.z.zhang@...ux.intel.com>,
	Chao Gao <chao.gao@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v11 07/23] 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 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.

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>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Reviewed-by: Chao Gao <chao.gao@...el.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 e07a03dbce6a..30d9d434c048 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -804,7 +804,6 @@ 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;
@@ -865,6 +864,8 @@ 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 9db246671885..9b45607f9b37 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 a018f327d5a1..dd984c6acae5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3992,16 +3992,17 @@ 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;
-- 
2.47.1