Subject: Re: [PATCH] x86_32: add support for 64 bit __get_user()
To: Benjamin LaHaise <bcrl@kvack.org>
References: <20160309172225.GN12913@kvack.org>
 <C08B33D1-0DC2-4470-8165-10DC62A6A94D@zytor.com>
 <20160309175016.GO12913@kvack.org>
Cc: Ingo Molnar <mingo@redhat.com>, Russell King <rmk@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <56E069FA.7070700@zytor.com>
Date: Wed, 9 Mar 2016 10:22:50 -0800
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On 03/09/2016 09:50 AM, Benjamin LaHaise wrote:
> On Wed, Mar 09, 2016 at 09:36:30AM -0800, H. Peter Anvin wrote:
>> On March 9, 2016 9:22:25 AM PST, Benjamin LaHaise <bcrl@kvack.org> wrote:
>>> The existing __get_user() implementation does not support fetching
>>> 64 bit values on 32 bit x86.  Implement this in a way that does not 
>>> generate any incorrect warnings as cautioned by Russell King.  Test 
>>> code available at http://www.kvack.org/~bcrl/x86_32-get_user.tar .  
> ...
>> Weird.  I could swear we had already fixed this a few years ago.
> 
> That surprised me as well, but Russell raised the fact that the approaches 
> previously tried on 32 bit architectures had caused various incorrect 
> compiler warnings for certain obscure cases -- see the code in test_module.c 
> in that URL that Russell provided to demonstrate the problem across all the 
> corner cases.

Oh, I see... I implemented it for put but not get... weird.  You may
want to look at the __inttype() macro defined earlier in this file; it
might be useful.

I presume you have already seen:

>> fs/select.c:710: Error: operand type mismatch for `movq'
>> fs/select.c:714: Error: incorrect register `%cx' used with `q' suffix
   fs/select.c:711: Error: operand type mismatch for `movq'
>> fs/select.c:715: Error: incorrect register `%si' used with `q' suffix
--
   fs/aio.c: Assembler messages:
>> fs/aio.c:1606: Error: operand type mismatch for `movq'
>> fs/aio.c:1610: Error: incorrect register `%si' used with `q' suffix

... which implies it used 16-bit registers for 64-bit operations when
compiling for 64 bits.

	-hpa