[PATCH v7 13/26] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Yang Weijiang]

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 modify XSS write access logic as:
If XSAVES is not enabled in the guest CPUID, forbid setting IA32_XSS msr
to anything but 0, even if the write is host initiated.

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
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 |  3 ++-
 arch/x86/kvm/cpuid.c            | 15 ++++++++++++++-
 arch/x86/kvm/x86.c              | 16 ++++++++++++----
 3 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 499bd42e3a32..f536102f1eca 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -756,7 +756,6 @@ 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;
@@ -812,6 +811,8 @@ 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 0351e311168a..1d9843b34196 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -275,7 +275,8 @@ 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);
+		best->ebx = xstate_required_size(vcpu->arch.xcr0 |
+						 vcpu->arch.ia32_xss, true);
 
 	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
 	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
@@ -312,6 +313,17 @@ static u64 vcpu_get_supported_xcr0(struct kvm_vcpu *vcpu)
 	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
 }
 
+static u64 vcpu_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 bool kvm_cpuid_has_hyperv(struct kvm_cpuid_entry2 *entries, int nent)
 {
 	struct kvm_cpuid_entry2 *entry;
@@ -358,6 +370,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	}
 
 	vcpu->arch.guest_supported_xcr0 = vcpu_get_supported_xcr0(vcpu);
+	vcpu->arch.guest_supported_xss = vcpu_get_supported_xss(vcpu);
 
 	kvm_update_pv_runtime(vcpu);
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f7d4cc61bc55..649a100ffd25 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3901,20 +3901,28 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			vcpu->arch.ia32_tsc_adjust_msr += adj;
 		}
 		break;
-	case MSR_IA32_XSS:
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
+	case MSR_IA32_XSS: {
+		/*
+		 * If KVM reported support of XSS MSR, even guest CPUID doesn't
+		 * support XSAVES, still allow userspace to set default value(0)
+		 * to this MSR.
+		 */
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
+		    !(msr_info->host_initiated && data == 0))
 			return 1;
 		/*
 		 * 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;
 		kvm_update_cpuid_runtime(vcpu);
 		break;
+	}
 	case MSR_SMI_COUNT:
 		if (!msr_info->host_initiated)
 			return 1;
-- 
2.27.0