PERFORCE change 50188 for review

[Julian Elischer]

Doug Rabson wrote:
> On Friday 02 April 2004 19:31, Julian Elischer wrote:
> 
>>Doug Rabson wrote:
>>
>>>On Fri, 2004-04-02 at 17:47, Julian Elischer wrote:
>>>
>>>>And the crowd goes wild...
>>>
>>>I'm not convinced that rtld is quite right yet. In particular,
>>>stuff like:
>>>	int __thread x[10];
>>>	&x[5];
>>>
>>>is probably broken.
>>>
>>>FWIW, our binutils doen't support the Sun abi at all...
>>
>>hmmm I wonder if it's planned or if we have to do it..
> 
> 
> Personally, I don't see the point. The GNU abi is smaller and faster and 
> will be better maintained by the gnu people over time. There isn't any 
> choice for any of the other platforms, including amd64 (which uses a 
> gnu-style abi with %fs:0 == %fs).

So we are screwed for amd64 basically.


But the reason the sun ABI axists is because on a PC using %gs as a segment 
register for thread identification, you cannot use the GNU model unless you
are using 1:1 threads.  You need to be able to change the place the pointer
points from userland. Obviously this requires a syscall as changing a [gl]dt
entry can not be done by a user process. This means that every context switch 
would require a syscall which defeats the entire point of using M:N threads.

The SUN API allows the destination of the %gs:0 to be changes at runtime by
the user this allowing the UTS to switch threads "on the fly" without
going back to the kernel.

Processors that have a "thread pointer" register are ok because the UTS
can just change it whenever it switches threads. Unfortunatly the X86
requires that we use a priviledged operation.
The only thing I can see as a possibility is if we make a special trap
into the kernel (bypassing all the normal syscall code)
that takes a single register as an argument and puts it into the
segment register descriptor pointed to by %gs after checking it VERY quickly,
and returns.. it may be possible to get in and out of the kernel quick enough 
that we don't lose performance.

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