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Sender: Arvind Sankar <niveditas98@gmail.com>
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Mon, 12 Oct 2020 16:29:34 -0400
To:     Borislav Petkov <bp@alien8.de>
Cc:     Uros Bizjak <ubizjak@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
Subject: Re: [GIT PULL] x86/asm updates for v5.10
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On Mon, Oct 12, 2020 at 08:55:47PM +0200, Borislav Petkov wrote:
> On Mon, Oct 12, 2020 at 08:41:32PM +0200, Uros Bizjak wrote:
> > On Mon, Oct 12, 2020 at 8:11 PM Linus Torvalds <
> > torvalds@linux-foundation.org> wrote:
> > 
> > > On Mon, Oct 12, 2020 at 4:06 AM Borislav Petkov <bp@suse.de> wrote:
> > > >
> > > > * Use XORL instead of XORQ to avoid a REX prefix and save some bytes in
> > > > the .fixup section, by Uros Bizjak.
> > >
> > > I think this one is actually buggy.
> > >
> > > For the 1-byte case, it does this:
> > >
> > >      __get_user_asm(x_u8__, ptr, retval, "b", "=q");
> > >
> > > and ends up doing "xorl" on a register that we told the compiler is a
> > > byte register (with that "=q")
> > >
> > > Yes, it uses "%k[output]" to turn that byte register into the word
> > > version of the register, but there's no fundamental reason why the
> > > register might not be something like "%ah".
> > >
> > 
> > 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.
> 
> Btw, did those get documented in the meantime? I can find them only in
> gcc sources:
> 
>    k --  likewise, print the SImode name of the register.
>    h -- print the QImode name for a "high" register, either ah, bh, ch or dh.

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers