Re: S3 regression related to XSA-471 patches

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Wed, Aug 06, 2025 at 12:46:36PM +0200, Marek Marczykowski-Górecki wrote:
> On Wed, Aug 06, 2025 at 12:36:56PM +0200, Jan Beulich wrote:
> > On 06.08.2025 12:23, Marek Marczykowski-Górecki wrote:
> > > We've got several reports that S3 reliability recently regressed. We
> > > identified it's definitely related to XSA-471 patches, and bisection
> > > points at "x86/idle: Remove broken MWAIT implementation". I don't have
> > > reliable reproduction steps, so I'm not 100% sure if it's really this
> > > patch, or maybe an earlier one - but it's definitely already broken at
> > > this point in the series. Most reports are about Xen 4.17 (as that's
> > > what stable Qubes OS version currently use), but I think I've seen
> > > somebody reporting the issue on 4.19 too (but I don't have clear
> > > evidence, especially if it's the same issue).
> > 
> > At the time we've been discussing the explicit raising of TIMER_SOFTIRQ
> > in mwait_idle_with_hints() a lot. If it was now truly missing, that imo
> > shouldn't cause problems only after resume, but then it may have covered
> > for some omission during resume. As a far-fetched experiment, could you
> > try putting that back (including the calculation of the "expires" local
> > variable)?
> 
> Sure, I'll try.

It appears this fixes the issue, at least in ~10 attempts so far
(usually I could reproduce the issue after 2-3 attempts).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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