Message-ID: <551C5BA5.9010407@zytor.com>
Date: Wed, 01 Apr 2015 13:57:09 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: Brian Gerst <brgerst@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        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>
Subject: Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions
References: <1427821211-25099-1-git-send-email-dvlasenk@redhat.com>	<1427821211-25099-7-git-send-email-dvlasenk@redhat.com>	<CAMzpN2iWm5r-n21yEJo11eNPhkFqcLo7OnPqGunJoRfKEkEhtg@mail.gmail.com>	<551BD241.4060207@redhat.com> <CA+55aFxuZvO=RjMGFm9Ng3ph1oKoARK4ZHdBotHsjpbKPs6qEQ@mail.gmail.com> <551C5A73.3050109@redhat.com>
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On 04/01/2015 01:52 PM, Denys Vlasenko wrote:
> On 04/01/2015 05:50 PM, Linus Torvalds wrote:
>> On Wed, Apr 1, 2015 at 4:10 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>>
>>> I did not know that. I was sure they are always zero extended.
>>
>> On all half-way modern cpu's they are. But on some older cpu's
>> (possibly just the original 386) the segment move instructions
>> basically are always 16-bit, and the operand size is ignored (so the
>> 32-bit version is just smaller and faster to decode, because it
>> doesn't have a 16-bit operand size prefix)
>>
>> Iirc, the same is true for the values pushed to memory on exceptions,
>> so the 'cs/ss' values on the exception stack may not be reliable in
>> the upper 16 bits.
>>
>> I don't remember if the same might be true of "pushl %Sseg". The intel
>> architecture manual says segment registers are zero-extended on push.
> 
> BTW, AMD64 docs do explicitly say that MOVs from segment registers
> to gpregs are zero-extending.
> 

For Intel processors it is true for Pentium Pro and later processors, as
far as I know.

	-hpa

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