[PATCH v3 14/21] KVM:VMX: Add a synthetic MSR to allow userspace to access GUEST_SSP

[Yang Weijiang <weijiang.yang@xxxxxxxxx>]

Introduce a host-only synthetic MSR, MSR_KVM_GUEST_SSP, so that the VMM
can read/write the guest's SSP, e.g. to migrate CET state.  Use a synthetic
MSR, e.g. as opposed to a VCPU_REG_, as GUEST_SSP is subject to the same
consistency checks as the PL*_SSP MSRs, i.e. can share code.

Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
 arch/x86/include/uapi/asm/kvm_para.h |  1 +
 arch/x86/kvm/vmx/vmx.c               | 15 ++++++++++++---
 arch/x86/kvm/x86.c                   |  4 ++++
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 6e64b27b2c1e..7af465e4e0bd 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -58,6 +58,7 @@
 #define MSR_KVM_ASYNC_PF_INT	0x4b564d06
 #define MSR_KVM_ASYNC_PF_ACK	0x4b564d07
 #define MSR_KVM_MIGRATION_CONTROL	0x4b564d08
+#define MSR_KVM_GUEST_SSP	0x4b564d09
 
 struct kvm_steal_time {
 	__u64 steal;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0ccaa467d7d3..72149156bbd3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2095,9 +2095,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_IA32_U_CET:
 	case MSR_IA32_PL3_SSP:
+	case MSR_KVM_GUEST_SSP:
 		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
 			return 1;
-		kvm_get_xsave_msr(msr_info);
+		if (msr_info->index == MSR_KVM_GUEST_SSP)
+			msr_info->data = vmcs_readl(GUEST_SSP);
+		else
+			kvm_get_xsave_msr(msr_info);
 		break;
 	case MSR_IA32_DEBUGCTLMSR:
 		msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
@@ -2413,15 +2417,20 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_IA32_U_CET:
 	case MSR_IA32_PL3_SSP:
+	case MSR_KVM_GUEST_SSP:
 		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
 			return 1;
 		if (is_noncanonical_address(data, vcpu))
 			return 1;
 		if (msr_index == MSR_IA32_U_CET && (data & GENMASK(9, 6)))
 			return 1;
-		if (msr_index == MSR_IA32_PL3_SSP && (data & GENMASK(2, 0)))
+		if ((msr_index == MSR_IA32_PL3_SSP ||
+		     msr_index == MSR_KVM_GUEST_SSP) && (data & GENMASK(2, 0)))
 			return 1;
-		kvm_set_xsave_msr(msr_info);
+		if (msr_index == MSR_KVM_GUEST_SSP)
+			vmcs_writel(GUEST_SSP, data);
+		else
+			kvm_set_xsave_msr(msr_info);
 		break;
 	case MSR_IA32_PERF_CAPABILITIES:
 		if (data && !vcpu_to_pmu(vcpu)->version)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2e3a39c9297c..baac6acebd40 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13642,6 +13642,10 @@ bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	    !guest_cpuid_has(vcpu, X86_FEATURE_IBT))
 		return false;
 
+	/* The synthetic MSR is for userspace access only. */
+	if (msr->index == MSR_KVM_GUEST_SSP)
+		return false;
+
 	if (msr->index == MSR_IA32_PL3_SSP &&
 	    !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK))
 		return false;
-- 
2.27.0