Re: [patch 3/3] KVM: x86: frequency change hypercalls

[Radim Krcmar]

2017-02-03 16:24-0200, Marcelo Tosatti:
> On Fri, Feb 03, 2017 at 06:40:34PM +0100, Radim Krcmar wrote:
>> You can make it one hypercall with an argument.
> 
> Fine.
> 
>> And the argument doesn't have to be enum {UP, DOWN, MAX, MIN}, but an
>> int, which would also allow you to do -2 steps.
> 
> Are you suggesting to have an integer to signify the number of steps up
> or down.

Yes.

>> A number over the capabilites of stepping would just map to MAX/MIN.
> 
> Then MAX == any positive value above the number of steps
>      MIN == any negative value below the negative of number of steps
> 
> Sure.
> 
>> Avoiding an absolute scale for interface simplifies migration, where the
>> guest cannot really depend much on this.  Except that calling it with
>> MIN (INT_MIN) will get the minimum and MAX (INT_MAX) the maximum
>> frequency.
> 
> Are you suggesting for the hypercall to return the maximum/minimum
> frequency if called with the highest integer and lowest negative integer 
> respectively? (That same hypercall).

No, I meant that we will guarantee that the guest will always get (the
CPU will be in) the minimal frequency when hypercall parameter is
INT_MIN and the maximal with INT_MAX -- just so the guest wouldn't lose
the ability which you provided by MIN and MAX hypercalls.

(We could also make a stronger assertion that there is never going to be
 more than INT_MAX steps, CPUs that run KVM will probably never have
 that fine frequency control.)

>> Plese explictly say in documentation that things like the number of
>> steps, which the guest can learn by doing MAX and then -1 until the
>> hypercall fails, is undefined and should not be depended upon.
> 
> Sure, because it fails over migration.
> 
>> Userspace might still want know the number of steps to avoid useless
>> hypercall -- I think we should return a different value when the limit
>> is reached, not just after the guest wants to go past it.
> 
> Are you suggesting to return a different value when going from 
> 
> max-1 -> max  
> and
> min+1 -> min
> 
> frequencies?

Yes.  Like you do now when going "up" from "max".
It saves one call of the hypercall.

> Fine.
> 
>> > +#endif
>> > +
>> >  	default:
>> >  		ret = -KVM_ENOSYS;
>> >  		break;
>> 
>> And thinking more about migration, userspace cannot learn the current
>> frequency (at least MIN/MAX), so the new host will just pick at random,
>> which will break userspace's expectations that it cannot increase or
>> decrease the frequency.  Is migration left for the future, because DPDK
>> doesn't migrate anyway?
>> 
>> Thanks.
> 
> The new host should start with the highest frequency always. Then
> the frequency tuning algorithm can reduce frequency afterwards.

That is not going to work on migration.

Suppose we do that and the CPU is in minimal frequency before the
migration.  This means that queue is below the threshold and userspace
knows that it is in minimum frequency (because we provide that
information when going down), so it doesn't trigger useless hypercalls.

After migration, the host would set frequency to maximum, but userspace
would still thing that it is minimal, so it would decrease it.

The only reason for this series -- power saving -- is lost.

> Migration is a desired feature for DPDK, so it should be supported
> (thats one reason why virtio-net drivers are used in the guest BTW).

Oh, nice,

thanks.