[PATCH v10 12/28] KVM: x86: Add a new save/restore flag for FRED metadata
From: Sohil Mehta
Date: Fri Sep 11 2026 - 17:43:12 EST
From: Xin Li <xin@xxxxxxxxx>
Introduce a new save/restore flag for FRED metadata (specifically, the
event nested flag and data) being saved and restored during VM save/
restore and live migration.
Signed-off-by: Xin Li <xin@xxxxxxxxx>
Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
---
v10:
- Advertise KVM_CAP_X86_FRED_EVENT only when KVM can virtualize FRED, and
return -EINVAL from KVM_ENABLE_CAP otherwise.
- Document the new return value in api.rst.
---
Documentation/virt/kvm/api.rst | 22 ++++++++++++++++++++++
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/include/uapi/asm/kvm.h | 1 +
arch/x86/kvm/x86.c | 18 ++++++++++++++++++
include/uapi/linux/kvm.h | 1 +
5 files changed, 43 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 4eb7e75a7473..22ef94f3eb3a 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1199,6 +1199,10 @@ The following bits are defined in the flags field:
triple_fault_pending field contains a valid state. This bit will
be set whenever KVM_CAP_X86_TRIPLE_FAULT_EVENT is enabled.
+- KVM_VCPUEVENT_VALID_FRED_STATE may be set to inform that the exception
+ state includes FRED state (specifically, the event nested flag and data).
+ This bit will be set whenever KVM_CAP_X86_FRED_EVENT is enabled.
+
ARM64:
^^^^^^
@@ -1301,6 +1305,11 @@ If KVM_CAP_X86_TRIPLE_FAULT_EVENT is enabled, KVM_VCPUEVENT_VALID_TRIPLE_FAULT
can be set in flags field to signal that the triple_fault field contains
a valid state and shall be written into the VCPU.
+If KVM_CAP_X86_FRED_EVENT is enabled, KVM_VCPUEVENT_VALID_FRED_STATE can be set
+in the flags field to inform that the exception state contains FRED state
+(specifically, the event nested flag and data), which shall be written into the
+VCPU.
+
ARM64:
^^^^^^
@@ -8979,6 +8988,19 @@ enabled, cmma can't be enabled anymore and pfmfi and the storage key
interpretation are disabled. If cmma has already been enabled or the
hpage_2g module parameter is not set to 1, -EINVAL is returned.
+7.48 KVM_CAP_X86_FRED_EVENT
+---------------------------
+
+:Architectures: x86
+:Parameters: args[0] whether feature should be enabled or not
+:Returns: 0 on success; -EINVAL if KVM cannot virtualize FRED.
+
+With this capability enabled, KVM allows exception save and restore operations
+to include FRED event context (specifically, the event nested flag and data).
+When injecting a FRED exception during VM entry, FRED event delivery relies
+on this information to select the correct event stack level and apply proper
+event data.
+
8. Other capabilities.
======================
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0fd5d4edeebb..dd68db17fdae 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1286,6 +1286,7 @@ struct kvm_arch {
bool has_mapped_host_mmio;
bool guest_can_read_msr_platform_info;
bool exception_payload_enabled;
+ bool exception_fred_state_enabled;
bool triple_fault_event;
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 1585ec804066..6a67832c59bb 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -331,6 +331,7 @@ struct kvm_reinject_control {
#define KVM_VCPUEVENT_VALID_SMM 0x00000008
#define KVM_VCPUEVENT_VALID_PAYLOAD 0x00000010
#define KVM_VCPUEVENT_VALID_TRIPLE_FAULT 0x00000020
+#define KVM_VCPUEVENT_VALID_FRED_STATE 0x00000040
/* Interrupt shadow states */
#define KVM_X86_SHADOW_INT_MOV_SS 0x01
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4b3681796c75..01198aba27cf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2288,6 +2288,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_PRE_FAULT_MEMORY:
r = tdp_enabled;
break;
+ case KVM_CAP_X86_FRED_EVENT:
+ r = kvm_cpu_cap_has(X86_FEATURE_FRED);
+ break;
case KVM_CAP_X86_APIC_BUS_CYCLES_NS:
r = kvm ? kvm->arch.apic_bus_cycle_ns : APIC_BUS_CYCLE_NS_DEFAULT;
break;
@@ -3022,6 +3025,8 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
| KVM_VCPUEVENT_VALID_SMM);
if (vcpu->kvm->arch.exception_payload_enabled)
events->flags |= KVM_VCPUEVENT_VALID_PAYLOAD;
+ if (vcpu->kvm->arch.exception_fred_state_enabled)
+ events->flags |= KVM_VCPUEVENT_VALID_FRED_STATE;
if (vcpu->kvm->arch.triple_fault_event) {
events->triple_fault.pending = kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu);
events->flags |= KVM_VCPUEVENT_VALID_TRIPLE_FAULT;
@@ -3036,6 +3041,7 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
| KVM_VCPUEVENT_VALID_SHADOW
| KVM_VCPUEVENT_VALID_SMM
| KVM_VCPUEVENT_VALID_PAYLOAD
+ | KVM_VCPUEVENT_VALID_FRED_STATE
| KVM_VCPUEVENT_VALID_TRIPLE_FAULT))
return -EINVAL;
@@ -3051,6 +3057,11 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
events->exception_has_payload = 0;
}
+ if (events->flags & KVM_VCPUEVENT_VALID_FRED_STATE) {
+ if (!vcpu->kvm->arch.exception_fred_state_enabled)
+ return -EINVAL;
+ }
+
if ((events->exception.injected || events->exception.pending) &&
(events->exception.nr > 31 || events->exception.nr == NMI_VECTOR))
return -EINVAL;
@@ -4038,6 +4049,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
kvm->arch.exception_payload_enabled = cap->args[0];
r = 0;
break;
+ case KVM_CAP_X86_FRED_EVENT:
+ r = -EINVAL;
+ if (!kvm_cpu_cap_has(X86_FEATURE_FRED))
+ break;
+ kvm->arch.exception_fred_state_enabled = cap->args[0];
+ r = 0;
+ break;
case KVM_CAP_X86_TRIPLE_FAULT_EVENT:
kvm->arch.triple_fault_event = cap->args[0];
r = 0;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 9fc8dfdfd65f..c5ffc4e7ddb1 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -998,6 +998,7 @@ struct kvm_enable_cap {
#define KVM_CAP_S390_VSIE_ESAMODE 248
#define KVM_CAP_S390_HPAGE_2G 249
#define KVM_CAP_ARM_PMU_V3_STRICT 250
+#define KVM_CAP_X86_FRED_EVENT 251
struct kvm_irq_routing_irqchip {
__u32 irqchip;
--
2.43.0