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SubjectRe: RFD: x32 ABI system call numbers
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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


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