Re: [PATCH] xen/console: do not drop serial output from the hardware domain

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 13, 2022 at 11:18:49AM +0200, Jan Beulich wrote:
> On 13.06.2022 11:04, Roger Pau Monné wrote:
> > On Mon, Jun 13, 2022 at 10:29:43AM +0200, Jan Beulich wrote:
> >> On 13.06.2022 10:21, Roger Pau Monné wrote:
> >>> On Mon, Jun 13, 2022 at 09:30:06AM +0200, Jan Beulich wrote:
> >>>> On 10.06.2022 17:06, Roger Pau Monne wrote:
> >>>>> Prevent dropping console output from the hardware domain, since it's
> >>>>> likely important to have all the output if the boot fails without
> >>>>> having to resort to sync_console (which also affects the output from
> >>>>> other guests).
> >>>>>
> >>>>> Do so by pairing the console_serial_puts() with
> >>>>> serial_{start,end}_log_everything(), so that no output is dropped.
> >>>>
> >>>> While I can see the goal, why would Dom0 output be (effectively) more
> >>>> important than Xen's own one (which isn't "forced")? And with this
> >>>> aiming at boot output only, wouldn't you want to stop the overriding
> >>>> once boot has completed (of which, if I'm not mistaken, we don't
> >>>> really have any signal coming from Dom0)? And even during boot I'm
> >>>> not convinced we'd want to let through everything, but perhaps just
> >>>> Dom0's kernel messages?
> >>>
> >>> I normally use sync_console on all the boxes I'm doing dev work, so
> >>> this request is something that come up internally.
> >>>
> >>> Didn't realize Xen output wasn't forced, since we already have rate
> >>> limiting based on log levels I was assuming that non-ratelimited
> >>> messages wouldn't be dropped.  But yes, I agree that Xen (non-guest
> >>> triggered) output shouldn't be rate limited either.
> >>
> >> Which would raise the question of why we have log levels for non-guest
> >> messages.
> > 
> > Hm, maybe I'm confused, but I don't see a direct relation between log
> > levels and rate limiting.  If I set log level to WARNING I would
> > expect to not loose _any_ non-guest log messages with level WARNING or
> > above.  It's still useful to have log levels for non-guest messages,
> > since user might want to filter out DEBUG non-guest messages for
> > example.
> 
> It was me who was confused, because of the two log-everything variants
> we have (console and serial). You're right that your change is unrelated
> to log levels. However, when there are e.g. many warnings or when an
> admin has lowered the log level, what you (would) do is effectively
> force sync_console mode transiently (for a subset of messages, but
> that's secondary, especially because the "forced" output would still
> be waiting for earlier output to make it out).

Right, it would have to wait for any previous output on the buffer to
go out first.  In any case we can guarantee that no more output will
be added to the buffer while Xen waits for it to be flushed.

So for the hardware domain it might make sense to wait for the TX
buffers to be half empty (the current tx_quench logic) by preempting
the hypercall.  That however could cause issues if guests manage to
keep filling the buffer while the hardware domain is being preempted.

Alternatively we could always reserve half of the buffer for the
hardware domain, and allow it to be preempted while waiting for space
(since it's guaranteed non hardware domains won't be able to steal the
allocation from the hardware domain).

For Xen it's not trivial to prevent messages from being dropped. At
least during Xen boot (system_state < SYS_STATE_active) we could also
active the sync mode and make the spin wait in __serial_putc process
softirqs.

Thanks, Roger.