Re: [PATCH v4 04/12] x86/mm: get INVLPGB count max from CPUID

[Rik van Riel <riel@surriel.com>]

On Mon, 2025-01-13 at 16:50 +0100, Jann Horn wrote:
> On Sun, Jan 12, 2025 at 4:55 PM Rik van Riel <riel@surriel.com>
> wrote:
> > +       /* Max number of pages INVLPGB can invalidate in one shot
> > */
> > +       if (boot_cpu_has(X86_FEATURE_INVLPGB)) {
> > +               cpuid(0x80000008, &eax, &ebx, &ecx, &edx);
> > +               invlpgb_count_max = (edx & 0xffff) + 1;
> 
> I assume the +1 is just a weird undocumented (or weirdly documented)
> encoding?
> https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24594.pdf
> says that field InvlpgbCountMax contains the "Maximum page count for
> INVLPGB instruction" and doesn't mention having to add 1 from what I
> can tell.
> 
The way I read the documentation, the number
passed in to invlpgb (and retrieved from cpuid)
corresponds to the number of extra pages
invalidated beyond the first page at the specified
address.

Things have not exploded on me invalidating
multiple pages at once in larger ranges, so I 
suspect my reading is right, but it would be
nice for one of the AMD people to confirm :)

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