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

[Zhao Liu <zhao1.liu@intel.com>]

> Please just make the new leaf have constant values based on just
> APX_F. We'll add the optional NCI/NDD/NF support if needed, i.e.
> never. :)

Maybe not never?

> > 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.

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