[PATCH v7 09/21] KVM: VMX: Add support for saving and restoring FRED MSRs

["Xin Li (Intel)" <xin@zytor.com>]

From: Xin Li <xin3.li@intel.com>

Introduce support for handling FRED MSR access requests, enabling both
host and guest to read and write FRED MSRs, which is essential for VM
save/restore and live migration, and allows userspace tools such as QEMU
to access the relevant MSRs.

Specially, intercept accesses to the FRED SSP0 MSR (IA32_PL0_SSP), which
remains accessible when FRED is enumerated even if CET is not.  This
ensures the guest value is fully virtual and does not alter the hardware
FRED SSP0 MSR.

Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
---

Change in v7:
* Intercept accesses to FRED SSP0, i.e., IA32_PL0_SSP, which remains
  accessible when FRED but !CET (Sean).

Change in v6:
* Return KVM_MSR_RET_UNSUPPORTED instead of 1 when FRED is not available
  (Chao Gao)
* Handle MSR_IA32_PL0_SSP when FRED is enumerated but CET not.

Change in v5:
* Use the newly added guest MSR read/write helpers (Sean).
* Check the size of fred_msr_vmcs_fields[] using static_assert() (Sean).
* Rewrite setting FRED MSRs to make it much easier to read (Sean).
* Add TB from Xuelian Guo.

Changes since v2:
* Add a helper to convert FRED MSR index to VMCS field encoding to
  make the code more compact (Chao Gao).
* Get rid of the "host_initiated" check because userspace has to set
  CPUID before MSRs (Chao Gao & Sean Christopherson).
* Address a few cleanup comments (Sean Christopherson).

Changes since v1:
* Use kvm_cpu_cap_has() instead of cpu_feature_enabled() (Chao Gao).
* Fail host requested FRED MSRs access if KVM cannot virtualize FRED
  (Chao Gao).
* Handle the case FRED MSRs are valid but KVM cannot virtualize FRED
  (Chao Gao).
* Add sanity checks when writing to FRED MSRs.
---
 arch/x86/include/asm/kvm_host.h |   5 ++
 arch/x86/kvm/vmx/vmx.c          |  45 ++++++++++++++
 arch/x86/kvm/x86.c              | 102 ++++++++++++++++++++++++++++++--
 3 files changed, 148 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 061c0cd73d39..bec644eec92f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1091,6 +1091,11 @@ struct kvm_vcpu_arch {
 #if IS_ENABLED(CONFIG_HYPERV)
 	hpa_t hv_root_tdp;
 #endif
+	/*
+	 * Stores the FRED SSP0 MSR when CET is not supported, prompting KVM
+	 * to intercept its accesses.
+	 */
+	u64 fred_ssp0_fallback;
 };
 
 struct kvm_lpage_info {
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 5f639fb3b44d..358410220cc2 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1387,6 +1387,18 @@ static void vmx_write_guest_kernel_gs_base(struct vcpu_vmx *vmx, u64 data)
 	vmx_write_guest_host_msr(vmx, MSR_KERNEL_GS_BASE, data,
 				 &vmx->msr_guest_kernel_gs_base);
 }
+
+static u64 vmx_read_guest_fred_rsp0(struct vcpu_vmx *vmx)
+{
+	return vmx_read_guest_host_msr(vmx, MSR_IA32_FRED_RSP0,
+				       &vmx->msr_guest_fred_rsp0);
+}
+
+static void vmx_write_guest_fred_rsp0(struct vcpu_vmx *vmx, u64 data)
+{
+	vmx_write_guest_host_msr(vmx, MSR_IA32_FRED_RSP0, data,
+				 &vmx->msr_guest_fred_rsp0);
+}
 #endif
 
 static void grow_ple_window(struct kvm_vcpu *vcpu)
@@ -1988,6 +2000,27 @@ int vmx_get_feature_msr(u32 msr, u64 *data)
 	}
 }
 
+#ifdef CONFIG_X86_64
+static const u32 fred_msr_vmcs_fields[] = {
+	GUEST_IA32_FRED_RSP1,
+	GUEST_IA32_FRED_RSP2,
+	GUEST_IA32_FRED_RSP3,
+	GUEST_IA32_FRED_STKLVLS,
+	GUEST_IA32_FRED_SSP1,
+	GUEST_IA32_FRED_SSP2,
+	GUEST_IA32_FRED_SSP3,
+	GUEST_IA32_FRED_CONFIG,
+};
+
+static_assert(MSR_IA32_FRED_CONFIG - MSR_IA32_FRED_RSP1 ==
+	      ARRAY_SIZE(fred_msr_vmcs_fields) - 1);
+
+static u32 fred_msr_to_vmcs(u32 msr)
+{
+	return fred_msr_vmcs_fields[msr - MSR_IA32_FRED_RSP1];
+}
+#endif
+
 /*
  * Reads an msr value (of 'msr_info->index') into 'msr_info->data'.
  * Returns 0 on success, non-0 otherwise.
@@ -2010,6 +2043,12 @@ int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_KERNEL_GS_BASE:
 		msr_info->data = vmx_read_guest_kernel_gs_base(vmx);
 		break;
+	case MSR_IA32_FRED_RSP0:
+		msr_info->data = vmx_read_guest_fred_rsp0(vmx);
+		break;
+	case MSR_IA32_FRED_RSP1 ... MSR_IA32_FRED_CONFIG:
+		msr_info->data = vmcs_read64(fred_msr_to_vmcs(msr_info->index));
+		break;
 #endif
 	case MSR_EFER:
 		return kvm_get_msr_common(vcpu, msr_info);
@@ -2242,6 +2281,12 @@ int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			vmx_update_exception_bitmap(vcpu);
 		}
 		break;
+	case MSR_IA32_FRED_RSP0:
+		vmx_write_guest_fred_rsp0(vmx, data);
+		break;
+	case MSR_IA32_FRED_RSP1 ... MSR_IA32_FRED_CONFIG:
+		vmcs_write64(fred_msr_to_vmcs(msr_index), data);
+		break;
 #endif
 	case MSR_IA32_SYSENTER_CS:
 		if (is_guest_mode(vcpu))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9930678f5a3b..c53fc235b8bd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -329,6 +329,9 @@ static const u32 msrs_to_save_base[] = {
 	MSR_STAR,
 #ifdef CONFIG_X86_64
 	MSR_CSTAR, MSR_KERNEL_GS_BASE, MSR_SYSCALL_MASK, MSR_LSTAR,
+	MSR_IA32_FRED_RSP0, MSR_IA32_FRED_RSP1, MSR_IA32_FRED_RSP2,
+	MSR_IA32_FRED_RSP3, MSR_IA32_FRED_STKLVLS, MSR_IA32_FRED_SSP1,
+	MSR_IA32_FRED_SSP2, MSR_IA32_FRED_SSP3, MSR_IA32_FRED_CONFIG,
 #endif
 	MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
 	MSR_IA32_FEAT_CTL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
@@ -1910,7 +1913,7 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
 		 * architecture. Intercepting XRSTORS/XSAVES for this
 		 * special case isn't deemed worthwhile.
 		 */
-	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
+	case MSR_IA32_PL1_SSP ... MSR_IA32_INT_SSP_TAB:
 		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
 			return KVM_MSR_RET_UNSUPPORTED;
 		/*
@@ -1925,6 +1928,52 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
 		if (index != MSR_IA32_INT_SSP_TAB && !IS_ALIGNED(data, 4))
 			return 1;
 		break;
+	case MSR_IA32_FRED_STKLVLS:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+			return KVM_MSR_RET_UNSUPPORTED;
+		break;
+	case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_RSP3:
+	case MSR_IA32_FRED_SSP1 ... MSR_IA32_FRED_CONFIG: {
+		u64 reserved_bits = 0;
+
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+			return KVM_MSR_RET_UNSUPPORTED;
+
+		if (is_noncanonical_msr_address(data, vcpu))
+			return 1;
+
+		switch (index) {
+		case MSR_IA32_FRED_CONFIG:
+			reserved_bits = BIT_ULL(11) | GENMASK_ULL(5, 4) | BIT_ULL(2);
+			break;
+		case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_RSP3:
+			reserved_bits = GENMASK_ULL(5, 0);
+			break;
+		case MSR_IA32_FRED_SSP1 ... MSR_IA32_FRED_SSP3:
+			reserved_bits = GENMASK_ULL(2, 0);
+			break;
+		default:
+			WARN_ON_ONCE(1);
+			return 1;
+		}
+
+		if (data & reserved_bits)
+			return 1;
+
+		break;
+	}
+	case MSR_IA32_PL0_SSP: /* I.e., MSR_IA32_FRED_SSP0 */
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+		    !guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+			return KVM_MSR_RET_UNSUPPORTED;
+
+		if (is_noncanonical_msr_address(data, vcpu))
+			return 1;
+
+		if (!IS_ALIGNED(data, 4))
+			return 1;
+
+		break;
 	}
 
 	msr.data = data;
@@ -1979,10 +2028,19 @@ static int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 		if (!host_initiated)
 			return 1;
 		fallthrough;
-	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
+	case MSR_IA32_PL1_SSP ... MSR_IA32_INT_SSP_TAB:
 		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
 			return KVM_MSR_RET_UNSUPPORTED;
 		break;
+	case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_CONFIG:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+			return KVM_MSR_RET_UNSUPPORTED;
+		break;
+	case MSR_IA32_PL0_SSP: /* I.e., MSR_IA32_FRED_SSP0 */
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+		    !guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+			return KVM_MSR_RET_UNSUPPORTED;
+		break;
 	}
 
 	msr.index = index;
@@ -4275,6 +4333,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 #endif
 	case MSR_IA32_U_CET:
 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
+			WARN_ON_ONCE(msr != MSR_IA32_FRED_SSP0);
+			vcpu->arch.fred_ssp0_fallback = data;
+			break;
+		}
+
 		kvm_set_xstate_msr(vcpu, msr_info);
 		break;
 	default:
@@ -4628,6 +4692,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 #endif
 	case MSR_IA32_U_CET:
 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
+			WARN_ON_ONCE(msr_info->index != MSR_IA32_FRED_SSP0);
+			msr_info->data = vcpu->arch.fred_ssp0_fallback;
+			break;
+		}
+
 		kvm_get_xstate_msr(vcpu, msr_info);
 		break;
 	default:
@@ -4648,8 +4718,23 @@ static bool is_xstate_managed_msr(u32 index)
 {
 	switch (index) {
 	case MSR_IA32_U_CET:
-	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+	case MSR_IA32_PL1_SSP ... MSR_IA32_PL3_SSP:
 		return true;
+	case MSR_IA32_PL0_SSP:
+		/*
+		 * When CET is not supported, XRSTORS/XSAVES do not cover
+		 * MSR_IA32_PL0_SSP.  However, this MSR remains accessible
+		 * to a FRED guest.
+		 *
+		 * Return false to skip loading the guest FPU in __msr_io()
+		 * whenever CET is unsupported, regardless of FRED support.
+		 *
+		 * Note: if !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) but
+		 * kvm_cpu_cap_has(X86_FEATURE_SHSTK), this function returns
+		 * true and XSAVES/XRSTORS save and restore MSR_IA32_PL0_SSP,
+		 * even such a guest doesn't affect the hardware PL0 SSP MSR.
+		 */
+		return kvm_caps.supported_xss & XFEATURE_MASK_CET_KERNEL;
 	default:
 		return false;
 	}
@@ -7603,10 +7688,19 @@ static void kvm_probe_msr_to_save(u32 msr_index)
 		if (!kvm_cpu_cap_has(X86_FEATURE_LM))
 			return;
 		fallthrough;
-	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+	case MSR_IA32_PL1_SSP ... MSR_IA32_PL3_SSP:
 		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
 			return;
 		break;
+	case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_CONFIG:
+		if (!kvm_cpu_cap_has(X86_FEATURE_FRED))
+			return;
+		break;
+	case MSR_IA32_PL0_SSP: /* I.e., MSR_IA32_FRED_SSP0 */
+		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
+		    !kvm_cpu_cap_has(X86_FEATURE_FRED))
+			return;
+		break;
 	default:
 		break;
 	}
-- 
2.51.0