Re: page table lock patch V15 [0/7]: overview

Andi Kleen
2005-01-14T17:11:24Z

On Fri, Jan 14, 2005 at 09:08:54AM -0800, Christoph Lameter wrote:
> On Fri, 14 Jan 2005, Andi Kleen wrote:
>
> > > Looked at arch/i386/lib/mmx.c. It avoids the mmx ops in an interrupt
> > > context but the rest of the prep for mmx only saves the fpu state if its
> > > in use. So that code would only be used rarely. The mmx 64 bit
> > > instructions seem to be quite fast according to the manual. Double the
> > > cycles than the 32 bit instructions on Pentium M (somewhat higher on Pentium 4).
> >
> > With all the other overhead (disabling exceptions, saving register etc.)
> > will be likely slower. Also you would need fallback paths for CPUs
> > without MMX but with PAE (like Ppro). You can benchmark
> > it if you want, but I wouldn't be very optimistic.
>
> So the PentiumPro is a cpu with atomic 64 bit operations in a cmpxchg but
> no instruction to do an atomic 64 bit store or load although the
> architecture conceptually supports 64bit atomic stores and loads? Wild.

It can do 64bit x87 FP loads/stores. But I doubt that is what you're
looking for.

-Andi