linux-kernel - Re: RFD: x32 ABI system call numbers

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Date:	Thu, 01 Sep 2011 18:51:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Pedro Alves <pedro@...esourcery.com>
CC:	Avi Kivity <avi@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"H.J. Lu" <hjl.tools@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Kuo <rkuo@...eaurora.org>,
	Mark Salter <msalter@...hat.com>,
	Jonas Bonn <jonas@...thpole.se>,
	Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: RFD: x32 ABI system call numbers

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
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