linux-kernel - Re: [PATCH] x86: bring back rep movsq for user access on CPUs without ERMS

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Date:   Mon, 28 Aug 2023 20:32:13 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        bp@...en8.de
Subject: Re: [PATCH] x86: bring back rep movsq for user access on CPUs without ERMS

On 8/28/23, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Mon, 28 Aug 2023 at 11:04, Mateusz Guzik <mjguzik@...il.com> wrote:
>>
>> Other files do it (e.g., see __copy_user_nocache), but I have no
>> strong opinion myself.
>
> So the __copy_user_nocache() thing is a case of that second issue -
> see my comment about "some sane visually sensible grouping" of the
> numbers.
>
> Look closer, and you'll notice that they aren't actually sequential.
> They are of the form XY where the X is the grouping, and Y is the
> local number within that grouping.
>
> That case also comes with a fair amount of comments about each group
> for the extable entries.
>
> But yes, we also do have a number of thos e"sequential labels". See
> for example arch/x86/lib/getuser.S, where we then end up having all
> the exception handling at the end because it is mostly shared across
> cases. It's ugly.
>
> We also have a lot of ugly cases that probably shouldn't use numbers
> at all, eg csum_partial(). I think that goes back to some darker age
> when things like "assembly is so trivial that it doesn't need any
> fancy explanatory labels for code" was ok.
>
> See also arch/x86/lib/memmove_64.S for similar horrors. I wonder if it
> is a case of "use compiler to get almost the right code, then massage
> things manually". Nasty, nasty. That should use legible names, not
> random numbers.
>
> I also suspect some people really thought that the numbers need to be
> unique, and just didn't know to use local numbering.
>

That was bad example, I meant stuff was already *unique* in other
files and it is sequential in some of them. In the very func I'm
modifying here there is 0/1 followed by 2/3 pair already, so it looked
like the convention to follow.

Anyhow this is bullshit detail I'm not going to argue about, you made
your position clear and I see no problem adhering to it -- consider
this bit patched in v2.

Can we drop this aspect please ;)

-- 
Mateusz Guzik <mjguzik gmail.com>