Re: [RFC PATCH] KVM: optimize apic interrupt delivery

[Gleb Natapov <gleb@redhat.com>]

On Tue, Sep 11, 2012 at 12:29:06PM +0300, Avi Kivity wrote:
> On 09/10/2012 08:05 PM, Gleb Natapov wrote:
> > On Mon, Sep 10, 2012 at 07:17:54PM +0300, Gleb Natapov wrote:
> >> > > +	return 0;
> >> > > +}
> >> > > +
> >> > > +static inline int kvm_apic_set_id(struct kvm_lapic *apic, u8 id)
> >> > > +{
> >> > > +	apic_set_reg(apic, APIC_ID, id << 24);
> >> > > +	return recalculate_apic_map(apic->vcpu->kvm);
> >> > > +}
> >> > > +
> >> > > +static inline int kvm_apic_set_ldr(struct kvm_lapic *apic, u32 id)
> >> > > +{
> >> > > +	apic_set_reg(apic, APIC_LDR, id);
> >> > > +	return recalculate_apic_map(apic->vcpu->kvm);
> >> > > +}
> >> > > +
> >> > 
> >> > return value of these functions seems never checked.
> >> > 
> >> Yes, the problem is that we can do nothing about the failure if failure
> >> happens during guest write.
> 
> We can.  Return -ENOMEM all the way up to userspace.
> 
There is no userspace to return error to if error happens on guest MMIO
write. Unless you mean return it as a return value of ioctl(VM_RUN) in
which case it is equivalent of killing the guest. And this is not fair
to a guest who did nothing wrong to suffer from our stupid optimizations :)
Actually I am not sure that returning to userspace in the middle of an
IO that is handled by a kernel is well defined in KVM ABI.

> >> 
> > Actually I have an idea how to handle the error. Never return one. If
> > map cannot be allocated go slow path always. phys_map should be checked
> > for NULL during delivery in this case obviously.
> 
> That's better of course (though we have to beware of such tricks, but in
> this case the slow path is regularly exercised so it should keep working).
> 
Oh with Windows guests it has work to do for sure.

--
			Gleb.