Re: [PATCH 1/2] x86/efi: Simplify efi_arch_cpu() a little

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

On Wed Jul 24, 2024 at 6:42 AM BST, Jan Beulich wrote:
> On 23.07.2024 15:47, Alejandro Vallejo wrote:
> > On Mon Jul 22, 2024 at 11:18 AM BST, Andrew Cooper wrote:
> >> +    if ( (eax >> 16) != 0x8000 || eax < 0x80000000U )
> >> +        blexit(L"In 64bit mode, but no extended CPUID leaves?!?");
> > 
> > I'm not sure about the condition even for the old code. If eax had 0x90000000
> > (because new convention appeared 10y in the future), then there would be
> > extended leaves but we would be needlessly bailing out. Why not simply check
> > that eax < 0x80000001 in here?
>
> eax = 0x90000000 is in leaf group 0x9000, not in the extended leaf group
> (0x8000). The splitting into groups may not be written down very well,
> but you can see the pattern in e.g. groups 0x8086 and 0xc000 also being
> used (by non-Intel non-AMD hardware), without those really being extended
> leaves in the sense that 0x8000xxxx are.
>
> Jan

The code is checking for a number specifically in the extended group, but
that's the output of leaf 0x80000000 which is defined to be just that.

AMD: "The value returned in EAX provides the largest extended function number
      supported by the processor"

Intel: "Maximum Input Value for Extended Function CPUID Information."

Unless there are quirks I don't know about (I admit it's not unlikely) I just
don't see why this condition needs to be anything else than a check that the
maximum function number is bigger than any of the leaves we read further ahead.

If the number happens to start with 8000, that'd be fine; but there's no reason
to bail out if it was 8001. And even if there was, the exit message is
misleading as it's claiming there's no extended CPUID leaves when in reality an
unexpected max-extended-leaf was read off the base extended leaf.

Not that it matters a whole lot in practice because that's going to be within
range. But it feels like a needless complication of the check.

Regardless, as I said it's more of a comment on the previous code than it is
about this mechanical transformation.

Cheers,
Alejandro