linux-kernel - Re: [PATCH v2 3/6] KVM: arm64: Allow userspace to inject external instruction aborts

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Message-ID: <aHK7w4TTEm7a1mco@linux.dev>
Date: Sat, 12 Jul 2025 12:47:15 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Jiaqi Yan <jiaqiyan@...gle.com>
Cc: maz@...nel.org, 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,
	duenwen@...gle.com, rananta@...gle.com, jthoughton@...gle.com
Subject: Re: [PATCH v2 3/6] KVM: arm64: Allow userspace to inject external
 instruction aborts

On Fri, Jul 11, 2025 at 04:58:57PM -0700, Jiaqi Yan wrote:
> On Fri, Jul 11, 2025 at 12:42 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
> >
> > On Wed, Jun 04, 2025 at 05:08:58AM +0000, Jiaqi Yan wrote:
> > > From: Raghavendra Rao Ananta <rananta@...gle.com>
> > >
> > > When KVM returns to userspace for KVM_EXIT_ARM_SEA, the userspace is
> > > encouraged to inject the abort into the guest via KVM_SET_VCPU_EVENTS.
> > >
> > > KVM_SET_VCPU_EVENTS currently only allows injecting external data aborts.
> > > However, the synchronous external abort that caused KVM_EXIT_ARM_SEA
> > > is possible to be an instruction abort. Userspace is already able to
> > > tell if an abort is due to data or instruction via kvm_run.arm_sea.esr,
> > > by checking its Exception Class value.
> > >
> > > Extend the KVM_SET_VCPU_EVENTS ioctl to allow injecting instruction
> > > abort into the guest.
> > >
> > > Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> > > Signed-off-by: Jiaqi Yan <jiaqiyan@...gle.com>
> >
> > Hmm. Since we expose an ESR value to userspace I get the feeling that we
> > should allow the user to supply an ISS for the external abort, similar
> > to what we already do for SErrors.
> 
> Oh, I will create something in v3, by extending kvm_vcpu_events to
> something like:
> 
> struct {
>   __u8 serror_pending;
>   __u8 serror_has_esr;
>   __u8 ext_dabt_pending;
>   __u8 ext_iabt_pending;
>   __u8 ext_abt_has_esr;  // <= new
>   /* Align it to 8 bytes */
>   __u8 pad[3];
>   union {
>     __u64 serror_esr;
>     __u64 ext_abt_esr;  // <= new

This doesn't work. The ABI allows userspace to pend both an SError and
SEA, so we can't use the same storage for the ESR.

>   };
> } exception;
> 
> One question about the naming since we cannot change it once
> committed. Taking the existing SError injection as example, although
> the name in kvm_vcpu_events is serror_has_esr, it is essentially just
> the ISS fields of the ESR (which is also written in virt/kvm/api.rst).
> Why named after "esr" instead of "iss"? The only reason I can think of
> is, KVM wants to leave the room to accept more fields than ISS from
> userspace. Does this reason apply to external aborts? Asking in case
> if "iss" is a better name in kvm_vcpu_events, maybe for external
> aborts, we should use ext_abt_has_iss?

We will probably need to include more ESR fields in the future, like
ESR_ELx.ISS2. So let's just keep the existing naming if that's OK with
you.

Thanks,
Oliver