Re: [PATCH v2] x86/intel: workaround several MONITOR/MWAIT errata

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Apr 23, 2025 at 02:13:01PM +0100, Andrew Cooper wrote:
> On 23/04/2025 12:32 pm, Roger Pau Monne wrote:
> > There are several errata on Intel regarding the usage of the MONITOR/MWAIT
> > instructions, all having in common that stores to the monitored region
> > might not wake up the CPU.
> >
> > Fix them by forcing the sending of an IPI for the affected models.
> >
> > The Ice Lake issue has been reproduced internally on XenServer hardware,
> > and the fix does seem to prevent it.  The symptom was APs getting stuck in
> > the idle loop immediately after bring up, which in turn prevented the BSP
> > from making progress.  This would happen before the watchdog was
> > initialized, and hence the whole system would get stuck.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> > ---
> > Apollo and Lunar Lake fixes have not been tested, due to lack of hardware.
> 
> Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
> 
> > diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
> > index 420198406def..1dbf15b01ed7 100644
> > --- a/xen/arch/x86/acpi/cpu_idle.c
> > +++ b/xen/arch/x86/acpi/cpu_idle.c
> > @@ -441,8 +441,14 @@ void cpuidle_wakeup_mwait(cpumask_t *mask)
> >      cpumask_andnot(mask, mask, &target);
> >  }
> >  
> > +/* Force sending of a wakeup IPI regardless of mwait usage. */
> > +bool __ro_after_init force_mwait_ipi_wakeup;
> > +
> >  bool arch_skip_send_event_check(unsigned int cpu)
> >  {
> > +    if ( force_mwait_ipi_wakeup )
> > +        return false;
> > +
> 
> I don't especially like this.  The callers are a loop over all CPUs, and
> this can't be inlined/simplified automatically.

Hm, I can look into this later, I can make maybe turn
arch_skip_send_event_check into an inline.  Let me get this
committed first.

Thanks, Roger.