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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Feb 03, 2017 at 06:40:34PM +0100, Radim Krcmar wrote:
> 2017-02-02 15:47-0200, Marcelo Tosatti:
> > Implement min/max/up/down frequency change 
> > KVM hypercalls. To be used by DPDK implementation.
> > 
> > Also allow such hypercalls from guest userspace.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > 
> > ---
> > Index: kvm-pvfreq/arch/x86/kvm/x86.c
> > ===================================================================
> > --- kvm-pvfreq.orig/arch/x86/kvm/x86.c	2017-02-02 11:17:17.063756725 -0200
> > +++ kvm-pvfreq/arch/x86/kvm/x86.c	2017-02-02 11:17:17.822752510 -0200
> > @@ -6219,10 +6219,58 @@
> 
> [Here lived copy-paste.]
> 
> >  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> >  {
> >  	unsigned long nr, a0, a1, a2, a3, ret;
> >  	int op_64_bit, r;
> > +	bool cpl_check;
> >  
> >  	r = kvm_skip_emulated_instruction(vcpu);
> >  
> > @@ -6246,7 +6294,13 @@
> >  		a3 &= 0xFFFFFFFF;
> >  	}
> >  
> > -	if (kvm_x86_ops->get_cpl(vcpu) != 0) {
> > +	cpl_check = true;
> > +	if (nr == KVM_HC_FREQ_UP || nr == KVM_HC_FREQ_DOWN ||
> > +	    nr == KVM_HC_FREQ_MIN || nr == KVM_HC_FREQ_MAX)
> > +		if (vcpu->arch.allow_freq_hypercall == true)
> > +			cpl_check = false;
> > +
> > +	if (cpl_check == true && kvm_x86_ops->get_cpl(vcpu) != 0) {
> >  		ret = -KVM_EPERM;
> >  		goto out;
> >  	}
> > @@ -6262,6 +6316,21 @@
> >  	case KVM_HC_CLOCK_PAIRING:
> >  		ret = kvm_pv_clock_pairing(vcpu, a0, a1);
> >  		break;
> > +#ifdef CONFIG_CPU_FREQ_GOV_USERSPACE
> 
> CONFIG_CPU_FREQ_GOV_USERSPACE should be checked when enabling the
> capability.
> 
> > +	case KVM_HC_FREQ_UP:
> > +		ret = kvm_pvfreq_up(vcpu);
> > +		break;
> > +	case KVM_HC_FREQ_DOWN:
> > +		ret = kvm_pvfreq_down(vcpu);
> > +		break;
> > +	case KVM_HC_FREQ_MAX:
> > +		ret = kvm_pvfreq_max(vcpu);
> > +		break;
> > +	case KVM_HC_FREQ_MIN:
> > +		ret = kvm_pvfreq_min(vcpu);
> > +		break;
> 
> Having 4 hypercalls for this is an overkill.
> 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.

> 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).

Sure.

> 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?

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.

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).