Re: [PATCH v10 12/28] KVM: x86: Add a new save/restore flag for FRED metadata

[Binbin Wu <binbin.wu@linux.intel.com>]

On 9/12/2026 5:36 AM, Sohil Mehta wrote:
> From: Xin Li <xin@zytor.com>
> 
> 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.

Nit:
Would it be more natural to move this patch after patches 13 and 14?
The commit message and the document reference changes introduced
in later patches.

> 
> Signed-off-by: Xin Li <xin@zytor.com>
> Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
> ---
> 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;