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

[Alejandro Vallejo <agarciav@amd.com>]

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?

> 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?

Cheers,
Alejandro