Re: pcb offset into uarea

[Matt Thomas <matt%3am-software.com@localhost>]

On Feb 19, 2014, at 8:34 AM, David Holland <dholland-tech%netbsd.org@localhost> 
wrote:

> On Mon, Feb 17, 2014 at 09:25:49PM +0000, David Laight wrote:
>>>> I'm adding code to i386 and amd64 to save the ymm registers on process
>>>> switch - allowing userspace to use the AVX instructions.
>>>> [ensuing crap about the u area]
>>> 
>>> Why put it in the u area at all? It's a legacy concept of little
>>> continuing value.
>> 
>> Certainly most of the stuff that is in the pcb could be put into the lwp
>> structure. Apart form the fp save area it isn't even very big.
>> 
>> Putting the FP save area at the low address of the kernel stack pages
>> saves you having to worry about how bit it is.
>> (for 'stack grows down' systems).
> 
> On the other hand, varying the size of the kernel stack unpredictably
> doesn't sound like such a great plan. I suppose it's not by that much
> though.
> 
> I dunno, I just tend to think the u area is an anachronism and we'd be
> better off abolishing it.

For the aarch64 port, the only thing in the PCB is the fpu register set.
Everything else is in mdlwp.  Now the context switch code can ignore
the PCB entirely.  I've been thinking of doing something similar for
other ports i maintain.