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 <[email protected]>
Co-developed-by: Zhang Yi Z <[email protected]>
Signed-off-by: Zhang Yi Z <[email protected]>
Signed-off-by: Yang Weijiang <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Reviewed-by: Chao Gao <[email protected]>
Reviewed-by: Xiaoyao Li <[email protected]>
Tested-by: Mathias Krause <[email protected]>
Tested-by: John Allen <[email protected]>
Tested-by: Rick Edgecombe <[email protected]>
Signed-off-by: Chao Gao <[email protected]>
Reviewed-by: Binbin Wu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: yang-weijiang

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)

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;