linux-kernel - Re: [PATCH 01/26] mm: asi: Make some utility functions noinstr compatible

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Message-ID: <ab8ef5ef-f51c-4940-9094-28fbaa926d37@google.com>
Date: Tue, 29 Oct 2024 10:38:27 -0700
From: Junaid Shahid <junaids@...gle.com>
To: Brendan Jackman <jackmanb@...gle.com>, Borislav Petkov <bp@...en8.de>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>, Alexandre Chartre <alexandre.chartre@...cle.com>,
 Liran Alon <liran.alon@...cle.com>,
 Jan Setje-Eilers <jan.setjeeilers@...cle.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Mel Gorman <mgorman@...e.de>,
 Lorenzo Stoakes <lstoakes@...il.com>, David Hildenbrand <david@...hat.com>,
 Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>,
 Khalid Aziz <khalid.aziz@...cle.com>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Valentin Schneider <vschneid@...hat.com>, Paul Turner <pjt@...gle.com>,
 Reiji Watanabe <reijiw@...gle.com>, Ofir Weisse <oweisse@...gle.com>,
 Yosry Ahmed <yosryahmed@...gle.com>, Patrick Bellasi <derkling@...gle.com>,
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 Matteo Rizzo <matteorizzo@...gle.com>, Jann Horn <jannh@...gle.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 kvm@...r.kernel.org
Subject: Re: [PATCH 01/26] mm: asi: Make some utility functions noinstr
 compatible

On 10/25/24 6:21 AM, Brendan Jackman wrote:
> Hey Boris,
> 
> On Fri, 25 Oct 2024 at 13:41, Borislav Petkov <bp@...en8.de> wrote:
>>
>> On Fri, Jul 12, 2024 at 05:00:19PM +0000, Brendan Jackman wrote:
>>> +/*
>>> + * Can be used for functions which themselves are not strictly noinstr, but
>>> + * may be called from noinstr code.
>>> + */
>>> +#define inline_or_noinstr                                            \
>>
>> Hmm, this is confusing. So is it noinstr or is it getting inlined?
> 
> We don't care if it's getting inlined, which is kinda the point. This
> annotation means "you may call this function from noinstr code". My
> current understanding is that the normal noinstr annotation means
> "this function fundamentally mustn't be instrumented".
> 
> So with inline_or_noinstr you get:
> 
> 1. "Documentation" that the function itself doesn't have any problem
> with getting traced etc.
> 2. Freedom for the compiler to inline or not.
> 
>> I'd expect you either always inline the small functions - as you do for some
>> aleady - or mark the others noinstr. But not something in between.
>>
>> Why this?
> 
> Overall it's pretty likely I'm wrong about the subtlety of noinstr's
> meaning. And the benefits I listed above are pretty minor. I should
> have looked into this as it would have been an opportunity to reduce
> the patch count of this RFC!
> 
> Maybe I'm also forgetting something more important, perhaps Junaid
> will weigh in...

Yes, IIRC the idea was that there is no need to prohibit inlining for this class 
of functions.