Re: [PATCH 4/5] i386/cpu: Support APX CPUIDs

[Paolo Bonzini <pbonzini@redhat.com>]

On Wed, Nov 19, 2025 at 7:04 PM Florian Weimer <fweimer@redhat.com> wrote:
> This sentence (from APX spec rev.7) emphasizes the “Intel” vendor,
> > and its primary goal was to address and explain compatibility concern
> > for pre-enabling work based on APX spec v6. Prior to v7, APX included
> > NCI_NDD_NF by default, but this feature has now been separated from
> > basic APX and requires explicit checking CPUID bit.
> >
> > x86 ecosystem advisory group has aligned on APX so it may be possible
> > for other x86 vendors to implement APX without NCI_NDD_NF and this still
> > match with the APX spec.
>
> Well yes, but I doubt that the ecosystem will produce binaries
> specialized for APX *without* NDD.  It's fine to enumerate it
> separately, but that doesn't have any immediate consequences.  GCC makes
> it rather hard to build for APX without NDD, for example.  At least more
> difficult than building for AVX-512F without AVX-512VL.
>
> I just don't think software vendors are enthusiastic about having to
> create and support not one, but two builds for APX.  If NDD is optional
> in practice, it will not be possible to use it except for run-time
> generated code and perhaps very targeted optimizations because that
> single extra APX will just not use NDD.
>
> I feel like there has been a misunderstanding somewhere.

I totally agree and I think this addition to APX was very misguided,
no matter who proposed it.

However, for virtualization we probably should include this code no
matter how much I dislike it, because having to add the bit later
retroactively would be worse.

Paolo