linux-kernel - Re: [RFC PATCH v3 06/15] jump_label: Add forceful jump label type

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Message-ID: <xhsmhcyioa8lu.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Thu, 21 Nov 2024 16:51:09 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>
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Subject: Re: [RFC PATCH v3 06/15] jump_label: Add forceful jump label type

On 21/11/24 12:00, Peter Zijlstra wrote:
> On Wed, Nov 20, 2024 at 08:55:15AM -0800, Josh Poimboeuf wrote:
>> On Wed, Nov 20, 2024 at 03:57:46PM +0100, Peter Zijlstra wrote:
>> > On Wed, Nov 20, 2024 at 03:56:49PM +0100, Peter Zijlstra wrote:
>> >
>> > > But I think we can make the fall-back safer, we can simply force the IPI
>> > > when we poke at noinstr code -- then NOHZ_FULL gets to keep the pieces,
>> > > but at least we don't violate any correctness constraints.
>> >
>> > I should have read more; that's what is being proposed.
>>
>> Hm, now I'm wondering what you read, as I only see the text poke IPIs
>> being forced when the caller sets force_ipi, rather than the text poke
>> code itself detecting a write to .noinstr.
>
> Right, so I had much confusion and my initial thought was that it would
> do something dangerous. Then upon reading more I see it forces the IPI
> for these special keys -- with that force_ipi thing.
>
> Now, there's only two keys marked special, and both have a noinstr
> presence -- the entire reason they get marked.
>
> So effectively we force the IPI when patching noinstr, no?
>
> But yeah, this is not quite the same as not marking anything and simply
> forcing the IPI when the target address is noinstr.
>
> And having written all that; perhaps that is the better solution, it
> sticks the logic in text_poke and ensure it automagically work for all
> its users, obviating the need for special marking.
>

Okay so forcing the IPI for .noinstr patching lets us get rid of all the
force_ipi faff; however I would still want the special marking to tell
objtool "yep we're okay with this one", and still get warnings when a new
.noinstr key gets added so we double think about it.