[PATCH v3 07/21] 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 current required storage size for all
features enabled via XCR0 | XSS so that guest can allocate correct xsave
buffer.

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/kvm/cpuid.c | 7 +++++--
 arch/x86/kvm/x86.c   | 6 ++++--
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 123bf8b97a4b..cbb1b8a65502 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -277,8 +277,11 @@ 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 &&
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 33a780fe820b..ab3360a10933 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3776,8 +3776,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		 */
 		if (data & ~kvm_caps.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