[PATCH v8 09/21] KVM: VMX: Add support for saving and restoring FRED MSRs

Xin Li (Intel)
2025-10-14T01:10:36Z

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 | 85 +++++++++++++++++++++++++++++++--
3 files changed, 132 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 48598d017d6f..43a18e265289 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1092,6 +1092,11 @@ 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 3ea660c0481c..f797e6b12721 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1386,6 +1386,18 @@ 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)
@@ -1987,6 +1999,27 @@ 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.
@@ -2009,6 +2042,12 @@ 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);
@@ -2241,6 +2280,12 @@ 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 42ecd093bb4c..80c490e71853 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -331,6 +331,9 @@ 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,
@@ -1919,7 +1922,7 @@ 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;
/*
@@ -1934,6 +1937,52 @@ 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;
@@ -1988,10 +2037,19 @@ 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;
@@ -4316,6 +4374,12 @@ 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:
@@ -4669,6 +4733,12 @@ 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:
@@ -7712,10 +7782,19 @@ 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