Re: RFD: x32 ABI system call numbers

[Andy Lutomirski <luto@MIT.EDU>]

On 08/26/2011 07:39 PM, H. Peter Anvin wrote:
> On 08/26/2011 04:13 PM, Linus Torvalds wrote:
>>>
>>> The extra bit would be masked off and only affect device drivers like
>>> input which relies on is_compat().
>>
>> So a couple of questions:
>>
>>   - why do we need another system call model at all?
>
> We think we can get more performance for a process which doesn't need
> more than 4 GiB of virtual address space by allowing them to keep
> pointers 4 bytes long, while still giving them the advantage of 16
> 64-bit registers, PC-relative addressing, and so on.  Furthermore, there
> are users who seem more willing to port code known to not be 64-bit
> clean to x32 than to do a whole new port.
>
> If the question is "why not just thunk this in userspace", the answer is
> that we'd like to take advantage of the compat layer already in the kernel.
>
> If the question is "why not just use int $0x80" we actually did that in
> early prototyping, but SYSCALL64 is much faster.

This may be a dumb question, but:

Why not just set some high bit of rax to enable compat syscalls to be 
issued with the 64-bit SYSCALL instruction.  This could be done with 
zero overhead for normal 64-bit code (you can just adjust the existing 
system-call-number-too-high path) and the total kernel patch should be 
just a handful of lines.  Does x32 need any more kernel support than 
that?  You'll confuse strace on new binaries, but that shouldn't be a 
big deal.

Also, why do ioctls and userspace structs need any translation at all? 
This is a new ABI -- why not just teach x32 code to stick zeros in its 
structs in the appropriate places to use the 64-bit layout.

--Andy