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

[Avi Kivity]

Ingo Molnar wrote:
> * Avi Kivity <avi@xxxxxxxxxx> 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

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