Messages in this thread | | | From | Andi Kleen <> | Subject | Re: Sysenter crash with Nested Task Bit set | Date | Mon, 18 Sep 2006 18:39:45 +0200 |
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On Monday 18 September 2006 18:12, Benjamin LaHaise wrote: > On Mon, Sep 18, 2006 at 05:29:23PM +0200, Andi Kleen wrote: > > > - asm volatile("pushl %%ebp\n\t" \ > > > + asm volatile("pushfl\n\t" /* Save flags */ \ > > > + "pushl %%ebp\n\t" \ > > > > We used to do that pushfl/popfl some time ago, but Ben removed it because > > it was slow on P4. Ok, nobody thought of that case back then. > > It's the pushfl that will be slow on any OoO CPU, as it has dependancies on > any previous instructions that modified the flags, which ends up bringing > all of the memory ordering dependancies into play. Doing a popfl to set the > flags to some known value is much less expensive.
Yes it's never fast, but on basically all non P4 CPUs it is still fast enough to not be a problem. I suppose it causes a trace cache flush or something like that there.
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