Re: [PATCH v6 00/90] x86: Introduce a centralized CPUID data model

[Borislav Petkov]

On Wed, May 06, 2026 at 11:57:20PM +0200, Ahmed S. Darwish wrote:
> > Second, the number of words reported in 0001h and 0002h
> > should be enumerated, in case the list grows, and so that a
> > program can tell between all-zero and not-present. (ECX>0
> > sub leaves, basically.)
> >
> > Since 0001h has already used up all four registers, it can't
> > report a max sub leaf in EAX – so maybe e.g. 0000h EBX
> > has to do the job instead: it could do e.g. two byte fields, to
> > report the number of words instead, i.e. 0000h BL => 0001h
> > words, and 0001h BH => 0002h words. (Or wider fields... if
> > you expect more future growth for words.)
> >
> 
> For both CPUID(0x4c780001) and CPUID(0x4c780002), I can still reserve
> their EAX for that.  Something like:
> 
> <leaf id="0x4c780001">
>   <subleaf id="0">

Can we slow down here please?

If we're going to deal with maxleaves, then I'd say that ship has already
sailed looong time ago.

Because up until now there's no maxleaf for those Linux-defined words. So you
*still* have to know which kernel you're looking at to know which words are
defined, and which words are reserved zeros.

The only thing you have is the database we're building here.

And since it'll be append-only for the Linux leaves, if it is not in the
database, it does not exist.

If you want to use it and decypher feature words, you still need to know which
kernel we're talking about. Because cpufeatures.h has the number of
Linux-specific words which this particular kernel knows about.

And if we're going to differentiate between defined and reserved, then we need
to save the kernel version from which onwards those bits actually have
a meaning.

And that would be ugly.

So if you see word X, bit Y set, then that is a synthetic word bit and it is
enabled. If it is not set, it is not present or not enabled but again, without
kernel version you don't know anyway.

Thx.

-- 
Regards/Gruss,
    Boris.

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