Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM

[Avi Kivity <avi@redhat.com>]

Ingo Molnar wrote:
> * Avi Kivity <avi@redhat.com> wrote:
>
>   
>>> I.e. the 3000 cycles value itself could be eliminated as well. 
>>> (with just a common-sense max of say 100,000 cycles enforced)
>>>       
>> Yeah, though that has a much smaller effect as it's only 
>> responsible for a few microseconds of spinning.
>>     
>
> 3000 cycles would be 1-2 usecs. Isnt the VM exit+entry cost still in 
> that range?
>   

It's 3000 executions of rep nop, so you need to account for the entire 
spinlock loop body.

The Linux spinlock is

             "1:\t"
             "cmpl %0, %2\n\t"
             "je 2f\n\t"
             "rep ; nop\n\t"
             "movzwl %1, %2\n\t"
             /* don't need lfence here, because loads are in-order */
             "jmp 1b\n"

5 instructions, maybe 2-3 cycles, not counting any special rep nop 
overhead.  Mark, any idea what the spin time is?

VM entry/exit is around 1us on the newer processors.

-- 
error compiling committee.c: too many arguments to function