Re: [Qemu-devel] Re: [PATCH 26/35] kvm: Eliminate KVMState arguments

[Anthony Liguori]

On 01/11/2011 08:18 AM, Avi Kivity wrote:
> On 01/11/2011 04:00 PM, Anthony Liguori wrote:
> > On 01/11/2011 03:01 AM, Avi Kivity wrote:
> > > On 01/10/2011 10:23 PM, Anthony Liguori wrote:
> > > > > > I don't see how ioapic, pit, or pic have a system scope.
> > > > > They are not bound to any CPU like the APIC which you may have in 
> > > > > mind.
> > > >
> > > > And none of the above interact with KVM.
> > >
> > > They're implemented by kvm.  What deeper interaction do you have in 
> > > mind?
> >
> > The emulated ioapic/pit/pic do not interact with KVM at all.
>
> How can they "not interact" with kvm if they're implemented by kvm?
>
> I really don't follow here.

"emulated ioapic/pit/pic" == versions implemented in QEMU.  That's what 
I'm trying to say.  When not using the KVM versions of the devices, 
there are no interactions with KVM.

> > The KVM versions should be completely separate devices.
>
> Why?

Because the KVM versions are replacements.

> > > I don't see why.  Those are just two different implementations for 
> > > the same guest visible device.
> >
> > Right, they should appear the same to the guest but the fact that 
> > they're two different implementations should be reflected in the 
> > device tree.
>
> Why?
>
> To move beyond single-word questions, what is the purpose of the 
> device tree?  In my mind, it reflects the virtual hardware.  What's 
> important is that we have a PIC, virtio network adapter, and IDE 
> disk.  Not that they're backed by kvm, vhost-net, and qcow2.

Let me give a very concrete example to illustrate my point.

One thing I have on my TODO is to implement catch-up support for the 
emulated devices.  I want to implement three modes of catch-up support: 
drop, fast, and gradual.  Gradual is the best policy IMHO but fast is 
necessary on older kernels without highres timers.  Drop is necessary to 
maintain compatibility with what we have today.

The kernel PIT only implements one mode and even if the other two were 
added, even the newest version of QEMU needs to deal with the fact that 
there's old kernels out there with PIT's that only do fast.

So how does this get exposed to management tools?  Do you check for 
drift-mode=fast and transparently enable the KVM pit?  Do you fail if 
anything but drift-mode=fast is specified?

We need to have the following mechanisms:

1) the ability to select an in-kernel PIT vs. a userspace PIT

2) an independent mechanism to configure the userspace PIT

3) an independent mechanism to configure the in-kernel PIT.

The best way to do this is to make the in-kernel PIT a separate device.  
Then we get all of this for free.

> > 2) a user can explicitly create either the emulated version of the 
> > device or the in-kernel version of the device (no need for 
> > -no-kvm-irqchip)
>
> -device ioapic,model=kernel vs. -device kvm-ioapic?
>
> Is it really important to do that? 110% of the time we want the kernel 
> irqchips.  The remaining -10% are only used for testing.

If model=kernel makes the support options different, then you end up 
introduce another layer of option validation.  By using the later form, 
you get to leverage the option validation of qdev plus it makes it much 
clearer to users what options are supported in what model because now 
the documentation is explicit about it.

> > 3) a user can pass parameters directly to the in-kernel version of 
> > the device that are different from the userspace version (like 
> > selecting different interrupt catch-up methods)
>
> -device pit,model=qemu,catchup=slew
>
> error: catchup=slew not supported in this model
>
> I'm not overly concerned about the implementation part.  Though I 
> think it's better to have a single implementation with kvm acting as 
> an accelerator, having it the other way is no big deal.  What I am 
> worried about is exposing it as a monitor and migration ABI.  IMO the 
> only important thing is the spec that the device implements, not what 
> piece of code implements it.

Just as we do in the PIT, there's nothing wrong with making the device's 
migration compatible.  I'm not entirely sure what your concerns about 
the monitor are but there's simply no way to hide the fact that a device 
is implemented in KVM at the monitor level.  But really, is this 
something that management tools want?  I doubt it.  I think they want to 
have ultimate control over what gets created with us providing a 
recommended set of defaults.

Regards,

Anthony Liguori