[PATCH v10 10/28] KVM: VMX: Add support for saving and restoring FRED MSRs
From: Sohil Mehta
Date: Fri Sep 11 2026 - 17:48:07 EST
From: "Xin Li (Intel)" <xin@xxxxxxxxx>
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 (Intel) <xin@xxxxxxxxx>
Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
---
v10:
- Fix a corner case identified by Chao Gao.
---
arch/x86/include/asm/kvm_host.h | 5 ++
arch/x86/include/asm/msr-index.h | 2 +
arch/x86/kvm/msrs.c | 89 ++++++++++++++++++++++++++++++--
arch/x86/kvm/vmx/vmx.c | 45 ++++++++++++++++
4 files changed, 138 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 8d3999772ac2..0fd5d4edeebb 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1032,6 +1032,11 @@ 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/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index ad899d9070e1..b386709515c4 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -64,6 +64,8 @@
#define MSR_IA32_FRED_SSP3 0x1d3 /* Level 3 shadow stack pointer */
#define MSR_IA32_FRED_CONFIG 0x1d4 /* Entrypoint and interrupt stack level */
+#define FRED_CONFIG_RESERVED (BIT_ULL(2) | GENMASK_ULL(5, 4) | BIT_ULL(11))
+
/* Intel MSRs. Some also available on other CPUs */
#define MSR_TEST_CTRL 0x00000033
#define MSR_TEST_CTRL_SPLIT_LOCK_DETECT_BIT 29
diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index 66fa7140d65d..9eabb7b02cb5 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -193,6 +193,9 @@ 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,
@@ -772,7 +775,7 @@ 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;
/*
@@ -787,6 +790,52 @@ 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 = FRED_CONFIG_RESERVED;
+ 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;
@@ -841,10 +890,19 @@ 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;
@@ -1833,7 +1891,15 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
#endif
case MSR_IA32_U_CET:
+ kvm_set_xstate_msr(vcpu, msr_info);
+ break;
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:
@@ -2186,7 +2252,15 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
#endif
case MSR_IA32_U_CET:
+ kvm_get_xstate_msr(vcpu, msr_info);
+ break;
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:
@@ -2656,10 +2730,19 @@ 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;
}
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b6b73ab9cfb7..362bd4dda298 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1489,6 +1489,18 @@ 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)
@@ -2140,6 +2152,27 @@ 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.
@@ -2162,6 +2195,12 @@ 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);
@@ -2394,6 +2433,12 @@ 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))
--
2.43.0