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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jun 12, 2024 at 01:12:43PM +0200, Paolo Bonzini wrote:
> On Wed, Jun 12, 2024 at 1:04 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > On Wed, Jun 12, 2024 at 01:55:20PM +0300, Alexander Monakov wrote:
> > > Hello,
> > >
> > > I'm sending straightforward reverts to recent patches that bumped minimum
> > > required x86 instruction set to SSE4.2. The older chips did not stop working,
> > > and people still test and use new software on older hardware:
> > > https://sourceware.org/bugzilla/show_bug.cgi?id=31867
> > >
> > > Considering the very minor gains from the baseline raise, I'm honestly not
> > > sure why it happened. It seems better to let distributions handle that.
> >
> > Indeed distros are opinionated about the x86_64 baseline they want
> > to target.
> >
> > While RHEL-9 switched to a x86_64-v2 baseline, Fedora has repeatedly
> > rejected the idea of moving to an x86_64-v2 baseline, wanting to retain
> > full backwards compat. So this assumption in QEMU is preventing the
> > distros from satisfying their chosen build target goals.
> 
> I didn't do this because of RHEL9, I did it because it's silly that
> QEMU cannot use POPCNT and has to waste 2% of the L1 d-cache to
> compute the x86 parity flag (and POPCNT was introduced at the same
> time as SSE4.2).
> 
> Intel x86_64-v2 processors have been around for about 15 years, AMD
> for a little less (2011). I'd rather hear from users about the
> usecases for running QEMU on such old processors before reverting, as
> this does not get in the way of booting/installing distros on old
> machines. Unless QEMU is run from within the installation media, which
> it isn't, requiring a particular processor family does not prevent
> Fedora from being installable on pre-v2 processors.

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.

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 ?


With regards,
Daniel
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