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> >>> Marek Marczykowski-Górecki 31.08.2021, 02:52 >>>
> 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
On Fri, Sep 03, 2021 at 10:28:35PM +0200, Ulrich Windl wrote:
> Hi!
>
>
> Can't it be disabled via kernel (grub) command line, too?
This is exactly "the second method" above.
> Also rumours say you can even disable it at runtime (and the threads will be
> migrated to other threads before).
> Occasionally some tools seem to have problems with HT being disabled (like
> "expecting 8 CPUS, but only found 4").
This is kind of similar issue as the one discussed here. That's why it's
better to disable HT in BIOS - to not show those 8 CPUs at all. But from
the OS level, we don't have other choice, and we prefer a secure
default - that's why we disable HT at Xen level, to provide safer option
regardless of what user has set in the BIOS.
PS Please don't top-post. And keep the mailing list in CC.
- --
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
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