Re: [PATCH v2 1/2] xen/x86: io_apic: Introduce a command line option to skip timer check

[Julien Grall <julien@xxxxxxx>]

Hi,

On 14/12/2023 10:10, Jan Beulich wrote:
> On 11.12.2023 13:23, Julien Grall wrote:
> > --- a/docs/misc/xen-command-line.pandoc
> > +++ b/docs/misc/xen-command-line.pandoc
> > @@ -2535,6 +2535,17 @@ pages) must also be specified via the tbuf_size 
> > parameter.
> >   ### tickle_one_idle_cpu
> >   > `= <boolean>`
> >   
> > +### pit-irq-works (x86)
> > +> `=<boolean>`
> > +
> > +> Default: `false`
> > +
> > +Disables the code which tests for broken timer IRQ sources. Enabling
> > +this option will reduce boot time on HW where the timer works properly.
> > +
> > +If the system is unstable when enabling the option, then it means you
> > +may have a broken HW and therefore the testing cannot be be skipped.
> > +
> >   ### timer_slop
> >   > `= <integer>`
> With the rename this now needs to move up to retain sorting.
Ok.

> > --- a/xen/arch/x86/io_apic.c
> > +++ b/xen/arch/x86/io_apic.c
> > @@ -57,6 +57,14 @@ bool __initdata ioapic_ack_forced;
> >   int __read_mostly nr_ioapic_entries[MAX_IO_APICS];
> >   int __read_mostly nr_ioapics;
> >   
> > +/*
> > + * The logic to check if the timer is working is expensive. So allow
> > + * the admin to bypass it if they know their platform doesn't have
> > + * a buggy timer.
> > + */
> > +static bool __initdata pit_irq_works;
> > +boolean_param("pit-irq-works", pit_irq_works);
> > +
> >   /*
> >    * Rough estimation of how many shared IRQs there are, can
> >    * be changed anytime.
> > @@ -1502,6 +1510,9 @@ static int __init timer_irq_works(void)
> >   {
> >       unsigned long t1, flags;
> >   
> > +    if ( pit_irq_works )
> > +        return 1;
> When the check is placed here, what exactly use of the option means is
> system dependent. I consider this somewhat risky, so I'd prefer if the
> check was put on the "normal" path in check_timer(). That way it'll
> affect only the one case which we can generally consider "known good",
> but not the cases where the virtual wire setups are being probed. I.e.
I am not against restricting when we allow skipping the timer check. But 
in that case, I wonder why Linux is doing it differently?
After all, this code is heavily borrowed from Linux. So shouldn't we 
follow what they are doing?
Cheers,

--
Julien Grall