linux-kernel - Re: [RFC v2 7/9] x86/mm: Introduce Remote Action Request

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Message-Id: <4009A0C6-CE5C-4197-9F48-3805059C214E@gmail.com>
Date: Tue, 20 May 2025 23:26:57 +0300
From: Nadav Amit <nadav.amit@...il.com>
To: Rik van Riel <riel@...riel.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
 the arch/x86 maintainers <x86@...nel.org>,
 kernel-team@...a.com,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 luto@...nel.org,
 peterz@...radead.org,
 Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
 "H. Peter Anvin" <hpa@...or.com>,
 Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [RFC v2 7/9] x86/mm: Introduce Remote Action Request

> On 20 May 2025, at 16:00, Rik van Riel <riel@...riel.com> wrote:
> 
>> Putting aside the rest of the code, I see you don’t call
>> should_flush_tlb().
>> I think it is worth mentioning in commit log or comment the rationale
>> behind
>> it (and maybe benchmarks to justify it).
>> 
>> 
> The long term plan here is to simply have the originating
> CPU included in the cpumask, and have it send a RAR
> request to itself.

That’s unrelated. I was referring to considering supporting
some sort of lazy TLB to eliminate sending RAR to cores that
do not care about it. Is there a cost of RAR to more cores than
needed? My guess is that there is one, and maybe in such cases
you would want actual IPI and special handling.