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

[Paolo Bonzini <pbonzini@redhat.com>]

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Il mer 19 nov 2025, 08:12 Zhao Liu <zhao1.liu@intel.com> ha scritto:

> > > Note, APX_NCI_NDD_NF is documented as always enabled for Intel
> > > processors since APX spec (revision v7.0). Now any Intel processor
> > > that enumerates support for APX_F (CPUID.(EAX=0x7, ECX=1).EDX[21])
> > > will also enumerate support for APX_NCI_NDD_NF.
>
> 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.
>

Oh, I was not aware of that. It is really ugly but I guess that's not our
choice. :/ If QEMU ever implements APX emulation it will have NC/NDD/NF
though...

Paolo


> If we default to setting this NCI_NDD_NF bit for APX, then in the future
> when we run into other vendors that don't support this feature, we'll not
> only have to make it optional again, but we'll also need to do fixes
> similar to the ARCH_CAPABILITIES situation - checking vendors, fixing
> compatibility issues, and all that stuff.
>
> Therefore, compared to default setting to constant, I think the optional
> NCI_NDD_NF now not only aligns with arch spec but also prevents future
> compatibility issues. :)
>
> Thanks,
> Zhao
>
>

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