Re: [RFC 00/12] arm64: Add support for TLBI domains

[Zi Yan <ziy@nvidia.com>]

On 5 Oct 2026, at 2:29, Will Deacon wrote:

> On Sun, Oct 04, 2026 at 02:06:48PM -0400, Zi Yan wrote:
>> On Thu Oct 1, 2026 at 7:06 AM EDT, Kristina Martšenko wrote:
>>> This series allows the kernel to use TLBI domains. TLBI domains (FEAT_TLBID) is
>>> an Arm architecture feature that enables a TLB invalidation to target a subset
>>> of CPUs instead of the whole system. It can improve performance on systems with
>>> many CPUs. TLBID is not yet part of the Arm Architecture Reference Manual but
>>> some of it is described in the latest XML [1].
>>>
>>> The goal of this series is to send TLBI commands only to the CPUs where a
>>> process has run (i.e. where its page tables have been installed). The last
>>
>> IIUC, mm_cpumask() is used to store this information on x86 and other
>> archs. I see you put this information in tlbid_domains of mm_context_t
>> from patch 9, although in the form of tlb id domains. I wonder why not
>> make arm64 also update mm_cpumask() like x86 and other archs and derive
>> the tlbid domains based on mm_cpumask() and a cpu-to-domain mapping
>> (added in patch 7).
>
> Historically, maintaining mm_cpumask() was a horrible source of contention
> when we looked at it in the past. Maybe that's no longer true, but it

Do you have a pointer to the cost you have in mind? LLM can only finds some
overhead reduction work done by Rik for x86[1][2].

[1] https://lore.kernel.org/all/20180716190337.26133-8-riel@surriel.com/T/#u
[2] https://lore.kernel.org/all/20241114152723.1294686-2-riel@surriel.com/

BTW, patch 9 is updating tlbid domains unconditionally. It can be optimized
in the same way as Rik did for x86 mm_cpumask(), read the tlbid domain first
and only update if CPU is in a new domain.

> would be good to see some numbers if we want to start keeping that thing
> accurate on arm64.


Best Regards,
Yan, Zi