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

[Jann Horn]

On Sun, Jan 12, 2025 at 4:55 PM Rik van Riel <riel@xxxxxxxxxxx> 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.