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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 12 Oct 2020 11:56:45 -0700
Message-ID: <CAHk-=whbEhjSHR7D=8Q_xARd=UKHGcz750LoT1BeRdgTR+6uSw@mail.gmail.com>
Subject: Re: [GIT PULL] x86/asm updates for v5.10
To:     Uros Bizjak <ubizjak@gmail.com>
Cc:     Borislav Petkov <bp@suse.de>, x86-ml <x86@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
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On Mon, Oct 12, 2020 at 11:41 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> GCC does not distinguish between %ah and %al and it is not possible to pass "%ah" to the assembly. To access the high part of the %ax register, %h modifier has to be used in the assembly template.

Do you know whether that's true for clang too, for example?

Also note that even if the _asm_ might get "%al", maybe the compiler
decided to use "%ah" for something else?

I have memories of gcc using the high registers at some point, but it
might have been some special case code - and it might also be very
historical.

[ Goes off and checks ]

In fact, I can still find gcc generating high register code, although
it's quite possible that yes, it's only peephole bit extract
instruction kind of use..

I also find that clang generates code that uses the high byte
registers, although again, that's not from any knowledge of clang
internals, and just by looking at my kernel image disassembly.

So yes, it _may_ all be just peepholes, but it's not obvious that this
is all safe.

                   Linus