[PATCH v1 1/3] KVM: arm64: Add ESR class to the hyp_enter hyp event

[Fuad Tabba]

Hi Vincent,

On Fri, 09 Oct 2026 14:02:41 +0100, Vincent Donnefort
<vdonnefort at google.com> wrote:

[...]
> > Aren't fixed trace points ABI? This changes the raw values of the
> > reason field (1 was hvc and is now WFx) and the strings it prints.
> > Keeping the old values and recording the EC in a new field would avoid
> > that.
>
> The event format is described precisely in events/hypervisor/hyp_enter/format.
> Any tooling should use that, so it doesn't seem like ABI to me. Also, this is
> behind NVHE_EL2_DEBUG. The only precedent I know is in kernel/sched/ where there
> are only tracepoints to avoid having trace events.
>
> Finally, we do not expose (yet :)) the raw interface. So this part is definitely
> not ABI.
>
> I tried to avoid having the EC separately to keep the event as small as
> possible.

Fair enough, with it behind NVHE_EL2_DEBUG and no raw interface yet.

[...]
> > This also replaces the PSCI, ERET_HOST and ERET_GUEST exit reasons
> > with a single ERET, which the commit message doesn't mention.
>
> ERET_HOST is now "to=host reason=eret" and ERET_GUEST "to=guest reason=ERET".
>
> This one here could be considered a bug fix and I could make it a separate
> patch?

A separate patch sounds good.

[...]
> > Could ESR_EL2 be read only when the event is enabled? The read happens
> > even when tracing is disabled or compiled out, so every non-IRQ guest
> > exit now does an extra one: with NVHE_EL2_DEBUG=n, __kvm_vcpu_run()
> > goes from two reads of ESR_EL2 to three.
>
> Hum, that's a good point. I was thinking about having a HYP_REASON_ESR that when
> sent actually read the ESR. But then handle_trap() would read it twice, unless
> this one still gives the esr directly.
>
> Alternatively, I thought of moving the esr_el2 = read_sysreg_el2(SYS_ESR) out of
> __fixup_guest_exit(). But it didn't look nice.
>
> WDYS?

HYP_REASON_ESR sounds good to me, with handle_trap() still passing the
EC it already has. HE_ASSIGN is only built at EL2 and runs after the
enabled check, so the read can go there.

[...]
> > Could this print the EC value when it isn't in the table?
> > kvm_arm_exception_class has no entry for ILL, BTI, SME or GCS among
> > others, so an ARM_EXCEPTION_IL exit (EC 0x0E) shows up as UNKNOWN_ESR.
>
> I can extend kvm_arm_exception_class. Having a dynamic format string is not
> possible here.

Sounds good, that also helps kvm_exit, which prints from the same table.

Cheers,
/fuad