Re: spapr watchdog vs watchdog_perform_action() / QMP watchdog-set-action

[Peter Maydell]

On Fri, 26 Jan 2024 at 20:49, Markus Armbruster <armbru@redhat.com> wrote:
>
> Peter Maydell <peter.maydell@linaro.org> writes:
>
> > Hi; one of the "bitesized tasks" we have listed is to convert
> > watchdog timers which directly call qemu_system_reset_request() on
> > watchdog timeout to call watchdog_perform_action() instead. This
> > means they honour the QMP commands that let the user specifiy
> > the behaviour on watchdog expiry:
> > https://www.qemu.org/docs/master/interop/qemu-qmp-ref.html#qapidoc-141
> > https://www.qemu.org/docs/master/interop/qemu-qmp-ref.html#qapidoc-129
> > (choices include reset, power off the system, do nothing, etc).
> >
> > There are only a few remaining watchdogs that don't use the
> > watchdog_perform_action() function. In most cases the change
> > is obvious and easy: just make them do that instead of calling
> > qemu_system_reset_request(SHUTDOWN_CAUSE_GUEST_RESET).
> >
> > However, the hw/watchdog/spapr_watchdog.c case is trickier. As
> > far as I can tell from the sources, this is a watchdog set up via
> > a hypercall, and the guest makes a choice of "power off, restart,
> > or dump and restart" for its on-expiry action.
> >
> > What should this watchdog's interaction with the watchdog-set-action
> > QMP command be? If the user says "do X" and the guest says "do Y",
> > which do we do? (With the current code, we always honour what
> > the guest asks for and ignore what the user asks for.)
>
> Gut reaction: when the user says "do X", the guest should not get a say.
> But one of the values of X could be "whatever the guest says".

Mmm. Slightly awkwardly, we don't currently distinguish between
"action is reset because the user never expressed a preference"
and "action is reset because the user specifically asked for that",
but I guess in theory we could make that distinction. (Conveniently
there is no QMP action for "query current watchdog-action state",
so we don't need to worry about reflecting that distinction in the
QMP interface if we make it.)

-- PMM