On Thu, 18 Aug 2022, Ben Hutchings wrote:

I suspect someone thought it would be a good idea to compile the kernel
for P4 only, as both PIII and Athlon XP processors lack the SSE2
instruction set.

That was a good guess, though we don't change the configuration like
that in stable updates.

Well, that happened for Firefox, it became SSE2 only when they switched
to a higher upstream version in a stable update. 78.15.0esr-1~deb11u1
is the last version that works on non SSE2 CPUs.

The RETbleed mitigations, which are not needed on these CPUs or even
functional on 32-bit kernels, interact with the Spectre v2 mitigations,
which *are* used on these CPUs.  And unfortunately the RETbleed
mitigations added some unconditional LFENCE instructions, which should
be conditional since they are part of SSE2.

As a temporary workaround, disabling the Spectre v2 mitigation with the
kernel parameter "nospectre_v2" should allow this kernel version to run
on older CPUs without SSE2.  We'll fix this properly in a later update.

Glad to know that was just a mistake. I will stay with 5.10.0-16 until
the next update shows up.

                Etienne Vogt (etienne.v...@obspm.fr)
                Observatoire de Paris-Meudon
                Service Informatique

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