Re: "Hijacking" a PCU.

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

On Dec 15, 2012, at 12:48 PM, David Laight wrote:

> On Sat, Dec 15, 2012 at 11:24:09AM -0800, Matt Thomas wrote:
>>> 
>>> On amd64 the safe area we currently have for SSE2 is 512 bytes.
>>> Add support for the 256 AVX instructions and it increases to 832.
>>> You really don't want to be allocating multiple such saved areas
>>> (per lwp) on the off chance the kernel might want to use the registers.
>> 
>> Since this is MD, you only need to save the register your kernel
>> MD code will be using.
> 
>>> 3) Saving and restoring a register may zero the high bits of an
>>>  extended version of that register.
>> 
>> That's an md problem.
> 
> Since you are trying to sort out an MI solution you need to be aware
> of the known MD problems - otherwise the framework won't work for it.

I disagree.  Just because an MD platform sucks and has extra overhead because 
of architectural issues doesn't mean the framework is broken.  You'd have that 
issue regardless of implementation method.

> If an x86 program is using the 256bit AVX instructions, and some
> kernel code wants to use one of the 128bit SSE registers, then the
> kernel code has to save the 256 bit register, not the 128bit one.
> (And next year, the register might be extended even further, requiring
> different save/restore instructions.)

The hijack is entirely MD, if the kernel MD code doesn't use hijack then it 
doesn't really matter.

> Effectively this means that a completely separate fpu save area is needed.
> You can't just save a couple of registers on stack.

Sucks to be x86.  But then is that really anything new?