On 9/3/21 11:41 PM, Marek Marczykowski-Górecki wrote:
[...]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.

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.
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.

Couldn't xen/qubes set a boot warning to users that rely only on (2) to
encourage more strongly to disable by BIOS (1)? That seems a logic
measure to me. best, Bernhard

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