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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 23 Oct 2020 14:11:19 -0700
Message-ID: <CAHk-=wg9L3EZk=cBjt5R3LkE8Y6swwOZ8sxhpQYcJO3Fj1wLbQ@mail.gmail.com>
Subject: Re: [PATCH] x86/uaccess: fix code generation in put_user()
To:     Peter Anvin <hpa@zytor.com>
Cc:     Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Oct 23, 2020 at 2:00 PM <hpa@zytor.com> wrote:
>
> There is no same reason to mess around with hacks when we are talking about dx:ax, though.

Sure there is.

"A" doesn't actually mean %edx:%eax like you seem to think.

It actually means %eax OR %edx, and then if given a 64-bit value, it
will use the combination (with %edx being the high bits).

So using "A" unconditionally doesn't work - it gives random behavior
for 32-bit (or smaller) types.

Or you'd have to cast the value to always be 64-bit, and have the
extra code generation.

IOW, an unconditional "A" is wrong.

And the alternative is to just duplicate things, and go back to the
explicit size testing, but honestly, I really think that's much worse
than relying on a documented feature of "register asm()" that gcc
_documents_ is for this kind of inline asm use.

So the "don't do pointless conditional duplication" is certainly a
very sane reason for the code.

            Linus