[x86/cpuid] Question regarding historic leaf 0x80000000 code

[Ahmed S. Darwish]

Hi hpa,

As you probably know by now, we're working on a centralized "CPUID table"
data model, on top of the cleanups at:

    https://lore.kernel.org/x86-cpuid/20250304085152.51092-1-darwi@xxxxxxxxxxxxx
    https://lore.kernel.org/x86-cpuid/20250319122137.4004-1-darwi@xxxxxxxxxxxxx
    https://lore.kernel.org/x86-cpuid/20250312143738.458507-1-darwi@xxxxxxxxxxxxx

The idea is to remove all the direct CPUID queries from the x86 code, and
access a pre-filled table instead (with caveats outside the scope of this
question.)

While changing the CPUID queries one by one, I've stumbled with:

    => arch/x86/kernel/cpu/common.c
    void get_cpu_cap(struct cpuinfo_x86 *c)
    {
	...
	/* AMD-defined flags: level 0x80000001 */
	eax = cpuid_eax(0x80000000);
	c->extended_cpuid_level = eax;

	if ((eax & 0xffff0000) == 0x80000000) {
		if (eax >= 0x80000001) {
			cpuid(0x80000001, &eax, &ebx, &ecx, &edx);

			c->x86_capability[CPUID_8000_0001_ECX] = ecx;
			c->x86_capability[CPUID_8000_0001_EDX] = edx;
		}
	}
	...
    }

You've contributed that snippet here:

    https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?h=2.4.0-test11pre5&id=67ad24e6d39c3

Do you remember what was the rationale for the "if (eax & 0xffff0000) ==
0x80000000" check?

You've also contriubted a similar check to head_32.S:

    https://lore.kernel.org/r/1258154897-6770-2-git-send-email-hpa@xxxxxxxxx

    /* Check if extended functions are implemented */
    movl $0x80000000, %eax
    cpuid
    /* Value must be in the range 0x80000001 to 0x8000ffff */
    subl $0x80000001, %eax
    cmpl $(0x8000ffff-0x80000001), %eax
    ja .Lenable_paging

So I would assume, it would be safe to have a similar check in our
centralized "system CPUID table" scanner.

In all cases, if you know more details, it would be great to know, so
that I add it in the centralized CPUID table patch queue changelog (in a
separate patch.)

Thanks a lot!

--
Ahmed S. Darwish
Linutronix GmbH