Re: S3 regression related to XSA-471 patches

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On August 11, 2025 3:16:46 PM GMT+02:00, Andrew Cooper 
<andrew.cooper3@xxxxxxxxxx> wrote:
>On 07/08/2025 6:29 pm, Marek Marczykowski-Górecki wrote:
>> 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).
>>
>
>Can you show the exact code which seems to have made this stable?

This patch: 
https://github.com/marmarek/qubes-vmm-xen/blob/7c9e9e312948c772d9a68090109964121c1d16fe/0001-DEBUG-S3.patch

>
>We discussed this in the x86 maintainers meeting, and our best guess is
>a timer that's not torn down or recreated properly on S3, but this is
>largely speculation.
>
>~Andrew