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

[Thomas J. Merritt]

|<><><><><> Original message from Peter Tattam  <><><><><>
|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.  ( http://www.x86-64.org/lists/discuss/msg02964.html ... 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.

The only way to get from long-mode back to legacy-mode is to reset the
processor.  It can be done in software but you will likely lose interrupts.
Attempting to switch out of long-mode by modifying EFER will just get you a #GP
fault.  You might want to read Volume 2 section 14.6.2.

TJ Merritt
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