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

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Message-ID: <CAG48ez0jCFE1f7JDVSeKT1XG+FDVMANWOBXB2gMAnntspv06Jg@mail.gmail.com>
Date: Mon, 13 Jan 2025 16:50:11 +0100
From: Jann Horn <jannh@...gle.com>
To: Rik van Riel <riel@...riel.com>, thomas.lendacky@....com
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, bp@...en8.de, 
	peterz@...radead.org, dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com, 
	nadav.amit@...il.com, kernel-team@...a.com, linux-mm@...ck.org, 
	akpm@...ux-foundation.org
Subject: Re: [PATCH v4 04/12] x86/mm: get INVLPGB count max from CPUID

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