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Date: Thu, 01 Sep 2011 18:51:35 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Pedro Alves <pedro@codesourcery.com>
CC: Avi Kivity <avi@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Christoph Hellwig <hch@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, "H.J. Lu" <hjl.tools@gmail.com>,
        Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Richard Kuo <rkuo@codeaurora.org>, Mark Salter <msalter@redhat.com>,
        Jonas Bonn <jonas@southpole.se>, Tobias Klauser <tklauser@distanz.ch>
Subject: Re: RFD: x32 ABI system call numbers
References: <4E582577.2060805@zytor.com> <4E5F88F8.2040008@redhat.com> <4E5F930A.6070006@zytor.com> <201109020149.10780.pedro@codesourcery.com>
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On 09/01/2011 05:49 PM, Pedro Alves wrote:
>>>
>>>       struct iovec
>>>       {
>>>           void __user *iov_base;    /* BSD uses caddr_t (1003.1g requires
>>> void *) */
>>>           __kernel_size_t iov_len; /* Must be size_t (1003.1g) */
>>>       } __attribute__((x32_abi_64));
>>>
>>>       typedef long time_t __attribute__((x32_abi_64));
>>>
>>> The x32_abi_64 attribute converts pointers and longs back to 64-bit and
>>> adjusts the alignment accordingly.  If we tag all userspace visible
>>> structures with this attribute, we can use the 64-bit ABI without changes.
>
> I would expect no new gcc extension to be needed for that -- there's the
> mode attribute (you can read DI as 64-bit):
>
>   typedef void * __kernel_ptr64 __attribute ((mode(DI)));
>
>   struct iovec
>   {
>     __kernel_ptr64 iov_base;
>     ...
>   };
>

Does that work for *writing*, too?  That might be a very useful little 
escape hatch for some particularly tight corners.

	-hpa