[Lguest] [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

[Ingo Molnar]

* Alexander van Heukelum <heukelum at fastmail.fm> wrote:

> > | > | Opteron    (cycles): 1024 / 1157 / 3527
> > | > | Xeon E5345 (cycles): 1092 / 1085 / 6622
> > | > | Athlon XP  (cycles): 1028 / 1166 / 5192
> > | > 
> > | > Xeon is defenitely out of luck :-)
> > | 
> > | it's still OK - i.e. no outrageous showstopper overhead anywhere in 
> > | that instruction sequence. The total round-trip overhead is what will 
> > | matter most.
> > | 
> > | 	Ingo
> > | 
> > 
> > Don't get me wrong please, I really like what Alexander have done!
> > But frankly six time slower is a bit scarying me.

the cost is 6 cycles instead of 1 cycles. In a codepath that takes 
thousands of cycles and is often cache-limited.

> Thanks again ;). Now it _is_ six times slower to do this tiny piece 
> of code... But please keep in mind all the activity that follows to 
> save the current data segment registers (the stack segment and code 
> segment are saved automatically), the general purpose registers and 
> to load most of the data segments with kernel-space values. And 
> looking at it now... do_IRQ is also not exactly trivial.
> 
> Also, I kept the information that is saved on the stack exactly the 
> same. If this is not a requirement, "push %cs" is what is left of 
> this expensive (6 cycle!) sequence. Even that could be unnecessary 
> if the stack layout can be changed... But I'ld like to consider that 
> separately.

we really want to keep the stack frame consistent between all the 
context types. We can do things like return-to-userspace-from-irq or 
schedule-from-irq-initiated-event, etc. - so crossing between these 
context frames has to be standard and straightforward.

	Ingo