linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>, x86-ml <x86@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	lkml <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] Improve memset
Date: Tue, 17 Sep 2019 13:45:20 -0700	[thread overview]
Message-ID: <CAHk-=wjDiDOcz2GHC88rV8gySCMZZko8PFW-ywJDkeY5n+je9Q@mail.gmail.com> (raw)
In-Reply-To: <20190917201021.evoxxj7vkcb45rpg@treble>

On Tue, Sep 17, 2019 at 1:10 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> Could it instead do this?
>
>         ALTERNATIVE_2("call memset_orig",
>                       "call memset_rep",        X86_FEATURE_REP_GOOD,
>                       "rep; stosb",             X86_FEATURE_ERMS)
>
> Then the "reverse alternatives" feature wouldn't be needed anyway.

That sounds better, but I'm a bit nervous about the whole thing
because who knows when the alternatives code itself internally uses
memset() and then we have a nasty little chicken-and-egg problem.

Also, for it to make sense to inline rep stosb, I think we also need
to just make the calling conventions for the alternative calls be that
they _don't_ clobber other registers than the usual rep ones
(cx/di/si). Otherwise one big code generation advantage of inlining
the thing just goes away.

On the whole I get the feeling that this is all painful complexity and
we shouldn't do it. At least not without some hard performance numbers
for some huge improvement, which I don't think we've seen.

Because I find the thing fascinating conceptually, but am not at all
convinced I want to deal with the pain in practice ;)

              Linus

  reply	other threads:[~2019-09-17 20:45 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-13  7:22 [RFC] Improve memset Borislav Petkov
2019-09-13  7:35 ` Ingo Molnar
2019-09-13  7:50   ` Borislav Petkov
2019-09-13  8:51 ` Rasmus Villemoes
2019-09-13  9:00 ` Linus Torvalds
2019-09-13  9:18   ` Rasmus Villemoes
2019-09-13 10:42     ` Borislav Petkov
2019-09-13 16:36       ` Borislav Petkov
2019-09-16  9:18         ` Rasmus Villemoes
2019-09-16 17:25           ` Linus Torvalds
2019-09-16 17:40             ` Andy Lutomirski
2019-09-16 21:29               ` Linus Torvalds
2019-09-16 23:13                 ` Andy Lutomirski
2019-09-16 23:26                   ` Linus Torvalds
2019-09-17  8:15             ` Borislav Petkov
2019-09-17 10:55             ` David Laight
2019-09-17 20:10 ` Josh Poimboeuf
2019-09-17 20:45   ` Linus Torvalds [this message]
2019-09-19 12:55     ` Borislav Petkov
2019-09-19 12:49   ` Borislav Petkov
2019-09-14  9:29 Alexey Dobriyan
2019-09-14 11:39 ` Borislav Petkov

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAHk-=wjDiDOcz2GHC88rV8gySCMZZko8PFW-ywJDkeY5n+je9Q@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=bp@alien8.de \
    --cc=jpoimboe@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).