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

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Date:	Thu, 01 Sep 2011 07:13:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Avi Kivity <avi@...hat.com>
CC:	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 06:30 AM, Avi Kivity wrote:
> On 08/31/2011 08:09 PM, H. Peter Anvin wrote:
>>>  I really think that "x32" should try to aim *VERY* hard at using the
>>>  64-bit system calls, and seeing itself as being a "32-bit application
>>>  in a 64-bit world".  That's not just true for time_t (which I think
>>>  should be 64-bit on anything new that expects to survive for any
>>>  amount of time), but in general.
>>
>> We're trying for it.  The things we're trying to avoid is to muck (too
>> much) with the compat layer for the mega-multiplex system calls like
>> ioctl.  We can't just use the 64-bit ioctl because ioctl structures
>> generally contain pointers.
>>
> 
>      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.
> 
> Issues:
> &my_iovec->iov_base yields something that is not a void ** (reads of a 
> 64-bit pointer decay to a 32-bit pointer, writes zero extend).
>    printf formats will break
>    if someone embeds an iovec in a structure, it will occupy more space 
> than expected
> 

Yes, the idea of a compiler extension was floated... however, we'd
prefer not to go there if at all possible.

	-hpa
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