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On Mon, Aug 30, 2021 at 05:39:40PM -0700, Andrew David Wong wrote:
> On 8/30/21 2:12 PM, haaber wrote:
> > 
> > > > Kind of answering my own question, but disabling hyperthreading
> > > > happened to
> > > > be a workaround for the resume from suspend issue.
> > > 
> > > But shouldn't hyperthreading have already been disabled ever since
> > > QSB-043?
> > > 
> > > https://www.qubes-os.org/news/2018/09/02/qsb-43/
> > > 
> > I admit that I missed that one as well. Shame on me. Is there some way
> > to detect active hyperthreading on boot && print out a big red warning ?
> > 
> > That seems a reasonable measure, especially for new-comers how cannot
> > reasonably be asked to read all old QSB's first :)
> > 
> I'm confused. I was under the impression that Qubes OS (after the QSB-043
> patches) automatically disables hyper-threading for you such that you don't
> have to know anything, do anything, or read any past QSBs.
> As QSB-043 explains, you would have had to follow special instructions to
> re-enable hyper-threading in Qubes 3.2, and no such instructions were
> provided for re-enabling it in Qubes 4.0 (since, as the QSB explains, it's
> never safe in that release), so I don't even know how'd you do it in that
> release.
> But perhaps I'm mistaken or misunderstanding the question.

There are (at least) two ways to disable hyper-threading:
1. In system BIOS (if there is such option)
2. In software - by disabling every second thread of each core.

The QSB-043 uses the second method. It has is drawbacks, as the logic to
bring up and down CPUs is quite complex. And yes, there are known
issues[1] affecting suspend. Disabling hyper-threading in BIOS, prevents
Xen from starting those secondary threads at all, and so it doesn't need
to bring them down.

[1] https://github.com/QubesOS/qubes-issues/issues/6066#issuecomment-901843312

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


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