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From: Arnd Bergmann <arnd@arndb.de>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Pedro Alves <pedro@codesourcery.com>, Avi Kivity <avi@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Jonas Bonn <jonas@southpole.se>, Tobias Klauser <tklauser@distanz.ch>
Subject: Re: RFD: x32 ABI system call numbers
Date: Fri, 02 Sep 2011 10:02:38 +0200
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On Thursday 01 September 2011 18:51:35 H. Peter Anvin wrote:
> 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.

I've tried to use that extension in other contexts without much success,
mostly I believe because gcc back-end support for it needs to be there
but wasn't at the time I tried. If the x32 back-end does this correctly,
you win.

A different gcc extension that might turn out to be useful here is the
named address space extension that lets you annotate a pointer to
be different from other pointers. On the SPU architecture we use this
for the destinction between local 18 bit pointers and 64-bit pointers
into the user process address space.

	Arnd