Message-ID: <CACw3F53cqiwtGyeJw+baS23sK3byenC8R5ddzW6Q1e_Bzk8tJA@mail.gmail.com>
Date: Tue, 21 Oct 2025 09:13:55 -0700
From: Jiaqi Yan <jiaqiyan@...gle.com>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: maz@...nel.org, oliver.upton@...ux.dev, duenwen@...gle.com, 
	rananta@...gle.com, jthoughton@...gle.com, vsethi@...dia.com, jgg@...dia.com, 
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com, 
	catalin.marinas@....com, will@...nel.org, pbonzini@...hat.com, corbet@....net, 
	shuah@...nel.org, kvm@...r.kernel.org, kvmarm@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 3/3] Documentation: kvm: new UAPI for handling SEA

On Mon, Oct 13, 2025 at 6:51 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>
>
>
> 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@...gle.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
>

Thanks for your quick review, Randy. I have queued fixes and am
waiting for reviews on other commits in this PATCH.

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