Re: [PATCH] x86/asm/entry/64: better check for canonical address

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> > Indeed, an IRET ought to be pretty cheap for same-ring interrupt 
> > returns in any case.
> 
> Unfortunately, it is not. Try attached program.
> 
> On this CPU, 1 ns ~= 3 cycles.
> 
> $ ./timing_test64 callret
> 10000 loops in 0.00008s = 7.87 nsec/loop for callret
> 100000 loops in 0.00076s = 7.56 nsec/loop for callret
> 1000000 loops in 0.00548s = 5.48 nsec/loop for callret
> 10000000 loops in 0.02882s = 2.88 nsec/loop for callret
> 100000000 loops in 0.18334s = 1.83 nsec/loop for callret
> 200000000 loops in 0.36051s = 1.80 nsec/loop for callret
> 400000000 loops in 0.71632s = 1.79 nsec/loop for callret
> 
> Near call + near ret = 5 cycles
> 
> $ ./timing_test64 lret
> 10000 loops in 0.00034s = 33.95 nsec/loop for lret
> 100000 loops in 0.00328s = 32.83 nsec/loop for lret
> 1000000 loops in 0.04541s = 45.41 nsec/loop for lret
> 10000000 loops in 0.32130s = 32.13 nsec/loop for lret
> 20000000 loops in 0.64191s = 32.10 nsec/loop for lret
> 
> push my_cs + push next_label + far ret = ~90 cycles
> 
> $ ./timing_test64 iret
> 10000 loops in 0.00344s = 343.90 nsec/loop for iret
> 100000 loops in 0.01890s = 188.97 nsec/loop for iret
> 1000000 loops in 0.08228s = 82.28 nsec/loop for iret
> 10000000 loops in 0.77910s = 77.91 nsec/loop for iret
> 
> This is the "same-ring interrupt return". ~230 cycles!  :(

Ugh, that's really expensive! Why is that so? Same-ring irqs are 
supposedly a lot simpler.

Thanks,

	Ingo