Re: [PATCH RFC V6 1/5] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks

[Avi Kivity <avi@redhat.com>]

On 04/29/2012 04:52 PM, Gleb Natapov wrote:
> On Sun, Apr 29, 2012 at 04:26:21PM +0300, Avi Kivity wrote:
> > On 04/29/2012 04:20 PM, Gleb Natapov wrote:
> > > > > This is too similar to kvm_irq_delivery_to_apic(). Why not reuse it. We
> > > > > can use one of reserved delivery modes as PV delivery mode. We will
> > > > > disallow guest to trigger it through apic interface, so this will not be
> > > > > part of ABI and can be changed at will.
> > > > >
> > > > 
> > > > I'm not thrilled about this.  Those delivery modes will eventually
> > > > become unreserved.  We can have a kvm_lookup_apic_id() that is shared
> > > > among implementations.
> > > > 
> > > This is only internal implementation. If they become unreserved we will
> > > use something else.
> > >
> > 
> > Yeah, I'm thinking of that time.  Why do something temporary and fragile?
> > 
> Why is it fragile? Just by unreserving the value Intel will not break
> KVM. Only when KVM will implement apic feature that unreserves the value
> we will have to change internal implementation and use another value,
> but this will be done by the same patch that does unreserving. The
> unreserving may even never happen. 

Some remains of that may leak somewhere.  Why not add an extra
parameter?  Or do something like

kvm_for_each_apic_dest(vcpu, apic_destination) {
      ...
}

That can be reused in both the apic code and pv kick.

> Meanwhile kvm_irq_delivery_to_apic()
> will likely be optimized to use hash for unicast delivery and unhalt
> hypercall will benefit from it immediately.

Overloading delivery mode is not the only way to achieve sharing.

-- 
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