Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 14 Feb 2003 13:01:30 +1100 (EST) | From | Peter Tattam <> | Subject | Re: [discuss] Re: [Bug 350] New: i386 context switch very slow compared to 2.4 due to wrmsr (performance) |
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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.
Peter
-- Peter R. Tattam peter@trumpet.com Managing Director, Trumpet Software International Pty Ltd Hobart, Australia, Ph. +61-3-6245-0220, Fax +61-3-62450210
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