Re: [discuss] Re: [Bug 350] New: i386 context switch very slow compared to 2.4 due to wrmsr (performance)

From: Peter Tattam (
Date: Thu Feb 13 2003 - 21:01:30 EST

On Thu, 13 Feb 2003, Eric Northup wrote:

> On Thursday 13 February 2003 07:14 pm, Peter Tattam wrote:
> > On Thu, 13 Feb 2003, Andi Kleen wrote:
> > > [Hmm, this is becomming a FAQ]
> > >
> > > > Switching in and out of long mode is evil enough that I don't think it
> > > > is worth it. And encouraging people to write good JIT compiling
> > >
> > > Forget it. It is completely undefined in the architecture what happens
> > > then. You'll lose interrupts and everything. Nothing for an operating
> > > system intended to be stable.
> > >
> > > I have no plans at all to even think about it for Linux/x86-64.
> [snip]
> >
> > The only other unknown quantity is the time it takes for the CPU to
> > enable/disable long mode, but with modern CPU speeds, the interrupt latency
> > may only be mildy affect by such a process, unless the CPU is broken in
> > some way. I see no discussion in the AMD manuals regarding the cost of the
> > mode switch, only what AMD engineers have hinted at.
> I think the real issue is that AMD neither recommends nor supports this
> strategy. ( ... there were
> better posts but I couldn't find them) People with real hardware can't talk
> about it right now, but it seems to me this is just begging to get hit by
> errata -- how much effore do you think team Hammer spent testing a subtle
> mode transition which is marked "Don't do that!" ?

well, I guess AMD need to come out & explicitly state this somewhere other than
on a mailing list. I wouldn't be only one tempted to see if it can be done,
and if it becomes "necessary" for some OSes, AMD will get locked into a
backward compatibility minefield. Anyone know what Windows 64 does about this
issue? If Microsoft considers that it is sufficient to warp the CPU for v86
emulation, it may just be a done deal.


Peter R. Tattam                  
Managing Director,    Trumpet Software International Pty Ltd
Hobart, Australia,  Ph. +61-3-6245-0220,  Fax +61-3-62450210

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