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Date: Wed, 1 Apr 2015 15:14:17 -0700
Message-ID: <CA+55aFweqq2bM7KKJkt1zOWMrDxfu-WVXmCD0Seut3y-o0Bf=w@mail.gmail.com>
Subject: Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Brian Gerst <brgerst@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Apr 1, 2015 at 1:52 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>
> BTW, AMD64 docs do explicitly say that MOVs from segment registers
> to gpregs are zero-extending.

Yeah, I think anything even *remotely* recent enough to do 64-bit does
zero-extending.

Even on the 32-bit side, anything that does register renaming is much
better off with zero-extension than with partial register writes.

And I found the "push" thing. It's actually documented:

  "When pushing a segment selector onto the stack, the Pentium 4,
Intel Xeon, P6 family, and Intel486 processors
decrement the ESP register by the operand size and then write 2 bytes.
If the operand size is 32-bits, the upper
two bytes of the write are not modified"

but I can't find any similar documentation for the "mov
Sreg->register" thing. So now I'm starting to doubt my own memory.

                          Linus
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