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

[Tom Lendacky]

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@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.
>>
> 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
>