Control: retitle -1 [i386] Unconditional LFENCE instructions in 
Control: tag -1 confirmed upstream
Control: found -1 5.18.14-1

On Wed, 2022-08-17 at 11:42 +0200, Etienne Vogt wrote:
> I can confirm that this bug also occurs on Athlon XP systems (Generic VIA 
> KT333 motherboard, CPU AMD Athlon(tm) XP 2600+) : kernel panic early on
> boot.
> 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.

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.


Ben Hutchings
I haven't lost my mind; it's backed up on tape somewhere.

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