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

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Message-ID: <73329c7c-0821-df9f-ecf5-880172888513@amd.com>
Date: Mon, 13 Jan 2025 16:53:46 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Rik van Riel <riel@...riel.com>, Jann Horn <jannh@...gle.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 1/13/25 15:08, Rik van Riel wrote:
> 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@...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.
>>
> 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 :)

That is correct.

Thanks,
Tom
>