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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 25 Sep 2019 13:23:27 -0700
Message-ID: <CAHk-=whSHi46gH-aE8bQsv7hvKDSHpWKQyLW3qF3caAB59cH+g@mail.gmail.com>
Subject: Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Aleksa Sarai <cyphar@cyphar.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        GNU C Library <libc-alpha@sourceware.org>,
        Linux API <linux-api@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Sep 25, 2019 at 12:43 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> FWIW, I would probably add a kernel-space analogue of that thing at the
> same time - check that an area is all-zeroes is not all that rare.

Hmm. Maybe.

> Another thing is that for s390 we almost certainly want something better
> than word-by-word.  IIRC, word-sized userland accesses really hurt there.
> It's nowhere near as critical as with strncpy_from_user(), but with the
> same underlying issue.

Well, s390 does have those magic "area" instructions, but part of why
it's expensive on s390 is that they haven't implemented the
"user_access_begin()/end()' stuff. I think s390 could use that to at
least minimize some of the costs.

With the common case presumably being just a couple of words, it migth
not be worth it doing anything more than that even on s390.

Interestingly (or perhaps not) if I read the internal s390
implementation correctly, they kind of _have_ that concept and they
use it internally. It's just that they call it "enable_sacf_uaccess()"
and "disable_sacf_uaccess()" instead.

           Linus