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

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

On Fri, Apr 25, 2025 at 01:36:42PM +0100, Alejandro Vallejo wrote:
> On Wed Apr 23, 2025 at 12:32 PM BST, 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.
> 
> Ugh... so this is what it was... Awesome having this madness fixed.
> 
> Do you happen to know if Linux has a similar fix in place?

It should have now:

https://lore.kernel.org/lkml/20250421192205.7CC1A7D9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

> > This would happen before the watchdog was initialized, and hence the
> > whole system would get stuck.
> 
> That's nasty. It was the misassumption that the watchdog was already
> running that had me going in circles thinking it was a lockup rather
> than a livelock. Oh, well.
> 
> I believe the kudos for finally being able to reproduce this goes to
> Frediano?

Yes, Frediano managed to get it to reproduce reliably in the lab
(maybe 1-2 hits per day), and afterwards we pushed the rate up by just
rebooting in Xen itself after AP bringup, once we knew the specific
hardware that exhibited the issue.

Regards, Roger.