Re: [PATCH 0/5] Reinstate ability to use Qemu on pre-SSE4.1 x86 hosts

[Paolo Bonzini <pbonzini@redhat.com>]

On Wed, Jun 12, 2024 at 1:38 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> This isn't anything to do with the distro installer. The use case is that
> the distro wants all its software to be able to run on the x86_64 baseline
> it has chosen to build with.

Sure, and they can patch the packages if their wish is not shared by
upstream. Alternatively they can live with the fact that not all users
will be able to use all packages, which is probably already the case.
Or drop QEMU, I guess. Has FeSCO ever expressed how strict they are
and which of the three options they'd pick?

Either way, this only affects either the QEMU maintainers for the
distro, or the users of QEMU. It's only if the installation media used
QEMU, that this change would be actively blocking usage of the distro
on old processors.

> If we want to use POPCNT in the TCG code, can we not do a runtime check
> and selectively build pieces of code with  __attribute__((target("popcnt"))),
> as we've done historically for the bufferiszero.c code, rather than
> changing the entire QEMU baseline ?

bufferiszero.c has a very quick check in front of the indirect call
and runs for several hundred clock cycles, so the tradeoff is
different there.

I guess that, because these helpers are called by TCG, you wouldn't
pay the price of the indirect call. However, adding all this
infrastructure for 13-15 year old CPUs is not very enthralling.

Paolo