Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> > I'm a big fan of simplifying things, but.
> >
> > SIMD registers were growing in x86, and they are going 
> > to grow again, this time four-fold in Intel MIC: from 
> > sixteen 256-bit registers to thirty two 512-bit 
> > registers.
> >
> > That's 2 kbytes of data. Just moving this data out 
> > to/from memory will take some time.
> >
> > And some people talk about 1024-bit registers 
> > already...
> >
> > Let's not completely remove lazy FPU saving code just 
> > yet. Maybe we'll be forced to reinstate it.
> 
> I'd prefer a different partial solution: encourage 
> everyone to clear the xstate before making syscalls 
> (using e.g. vzeroall).  In fact, maybe user code should 
> aggressively clear newly-unused xstate.

Also, xstate has various compaction features and could grow 
new ones in the future as well, should the xsave area 
become overly sparse: see xstate_comp_*[] et al in 
arch/x86/kernel/xsave.c.

This is the better, hardware driven, synchronous 
alternative to lazy, async register state save/restore, as 
it gets us similar benefits of not saving/restoring unused 
space, but avoids any async trap overhead.

Also ... with more and wider vector CPU registers context 
switches between different sets of registers are going to 
be inevitably more expensive, no matter what.

Thanks,

	Ingo