Re: [PATCH v4 3/3] Documentation: kvm: new UAPI for handling SEA

[Randy Dunlap <rdunlap@infradead.org>]

On 10/13/25 11:59 AM, Jiaqi Yan wrote:
> Document the new userspace-visible features and APIs for handling
> synchronous external abort (SEA)
> - KVM_CAP_ARM_SEA_TO_USER: How userspace enables the new feature.
> - KVM_EXIT_ARM_SEA: exit userspace gets when it needs to handle SEA
>   and what userspace gets while taking the SEA.
> 
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 61 ++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 6ae24c5ca5598..43bc2a1d78e01 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7272,6 +7272,55 @@ exit, even without calls to ``KVM_ENABLE_CAP`` or similar.  In this case,
>  it will enter with output fields already valid; in the common case, the
>  ``unknown.ret`` field of the union will be ``TDVMCALL_STATUS_SUBFUNC_UNSUPPORTED``.
>  Userspace need not do anything if it does not wish to support a TDVMCALL.
> +
> +::
> +		/* KVM_EXIT_ARM_SEA */
> +		struct {
> +  #define KVM_EXIT_ARM_SEA_FLAG_GPA_VALID   (1ULL << 0)
> +			__u64 flags;
> +			__u64 esr;
> +			__u64 gva;
> +			__u64 gpa;
> +		} arm_sea;
> +
> +Used on arm64 systems. When the VM capability KVM_CAP_ARM_SEA_TO_USER is
> +enabled, a VM exit is generated if guest causes a synchronous external abort
> +(SEA) and the host APEI fails to handle the SEA.
> +
> +Historically KVM handles SEA by first delegating the SEA to host APEI as there
> +is high chance that the SEA is caused by consuming uncorrected memory error.
> +However, not all platforms support SEA handling in APEI, and KVM's fallback
> +is to inject an asynchronous SError into the guest, which usually panics
> +guest kernel unpleasantly. As an alternative, userspace can participate into

                                                                           in

> +the SEA handling by enabling KVM_CAP_ARM_SEA_TO_USER at VM creation, after
> +querying the capability. Once enabled, when KVM has to handle the guest

                                                                     guest-
> +caused SEA, it returns to userspace with KVM_EXIT_ARM_SEA, with details
> +about the SEA available in 'arm_sea'.
> +
> +The 'esr' field holds the value of the exception syndrome register (ESR) while
> +KVM taking the SEA, which tells userspace the character of the current SEA,
   KVM takes

> +such as its Exception Class, Synchronous Error Type, Fault Specific Code and
> +so on. For more details on ESR, check the Arm Architecture Registers
> +documentation.
> +
> +The following values are defined for the 'flags' field

Above needs an ending like '.' or ':'.
(or maybe "::" depending how it is processed by Sphinx)

> +
> +  - KVM_EXIT_ARM_SEA_FLAG_GPA_VALID -- the faulting guest physical address
> +    is valid and userspace can get its value in the 'gpa' field.
> +
> +Note userspace can tell whether the faulting guest virtual address is valid
> +from the FnV bit in 'esr' field. If FnV bit in 'esr' field is not set, the
> +'gva' field hols the valid faulting guest virtual address.

               holds (or contains)> +
> +Userspace needs to take actions to handle guest SEA synchronously, namely in
> +the same thread that runs KVM_RUN and receives KVM_EXIT_ARM_SEA. One of the
> +encouraged approaches is to utilize the KVM_SET_VCPU_EVENTS to inject the SEA
> +to the faulting VCPU. This way, the guest has the opportunity to keep running
> +and limit the blast radius of the SEA to the particular guest application that
> +caused the SEA. Userspace may also emulate the SEA to VM by itself using the
> +KVM_SET_ONE_REG API. In this case, it can use the valid values from 'gva' and
> +'gpa' fields to manipulate VCPU's registers (e.g. FAR_EL1, HPFAR_EL1).
> +
>  ::
>  
>  		/* Fix the size of the union. */
> @@ -8689,6 +8738,18 @@ This capability indicate to the userspace whether a PFNMAP memory region
>  can be safely mapped as cacheable. This relies on the presence of
>  force write back (FWB) feature support on the hardware.
>  
> +7.45 KVM_CAP_ARM_SEA_TO_USER
> +----------------------------
> +
> +:Architecture: arm64
> +:Target: VM
> +:Parameters: none
> +:Returns: 0 on success, -EINVAL if unsupported.
> +
> +This capability, if KVM_CHECK_EXTENSION indicates that it is available, means
> +that KVM has an implementation that allows userspace to participate in handling
> +synchronous external abort caused by VM, by an exit of KVM_EXIT_ARM_SEA.
> +
>  8. Other capabilities.
>  ======================
>  

-- 
~Randy