[PATCH v4 04/20] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Yang Weijiang]

Update CPUID(EAX=0DH,ECX=1) when the guest's XSS is modified.
CPUID(EAX=0DH,ECX=1).EBX reports required storage size of
all enabled xstate features in XCR0 | XSS. Guest can allocate
sufficient xsave buffer based on the info.

Note, KVM does not yet support any XSS based features, i.e.
supported_xss is guaranteed to be zero at this time.

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>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/cpuid.c            | 20 ++++++++++++++++++--
 arch/x86/kvm/x86.c              |  8 +++++---
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 28bd38303d70..20bbcd95511f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -804,6 +804,7 @@ struct kvm_vcpu_arch {
 
 	u64 xcr0;
 	u64 guest_supported_xcr0;
+	u64 guest_supported_xss;
 
 	struct kvm_pio_request pio;
 	void *pio_data;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 7f4d13383cf2..0338316b827c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -249,6 +249,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_cpuid_entry2 *entries, int nent)
 	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
 }
 
+static u64 cpuid_get_supported_xss(struct kvm_cpuid_entry2 *entries, int nent)
+{
+	struct kvm_cpuid_entry2 *best;
+
+	best = cpuid_entry2_find(entries, nent, 0xd, 1);
+	if (!best)
+		return 0;
+
+	return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
+}
+
 static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
 				       int nent)
 {
@@ -276,8 +287,11 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
 
 	best = cpuid_entry2_find(entries, nent, 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);
+		     cpuid_entry_has(best, X86_FEATURE_XSAVEC))) {
+		u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
+
+		best->ebx = xstate_required_size(xstate, true);
+	}
 
 	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
 	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
@@ -325,6 +339,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.guest_supported_xcr0 =
 		cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
+	vcpu->arch.guest_supported_xss =
+		cpuid_get_supported_xss(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
 
 	/*
 	 * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4597ab3c811c..26edb8fa857a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3781,10 +3781,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		 * 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;
-		vcpu->arch.ia32_xss = data;
-		kvm_update_cpuid_runtime(vcpu);
+		if (vcpu->arch.ia32_xss != data) {
+			vcpu->arch.ia32_xss = data;
+			kvm_update_cpuid_runtime(vcpu);
+		}
 		break;
 	case MSR_SMI_COUNT:
 		if (!msr_info->host_initiated)
-- 
2.27.0