Re: [RFC] Unify KVM kernel-space and user-space code into a single project

Anthony Liguori
2010-03-18T16:11:43Z

On 03/18/2010 10:17 AM, Ingo Molnar wrote:
> * Anthony Liguori<anthony@codemonkey.ws> wrote:
>
>
>> On 03/18/2010 08:00 AM, Ingo Molnar wrote:
>>
>>>> [...] kvm in fact knows nothing about vga, to take your last example.
>>>> [...]
>>>>
>>> Look at the VGA dirty bitmap optimization a'ka the KVM_GET_DIRTY_LOG
>>> ioctl.
>>>
>>> See qemu/kvm-all.c's kvm_physical_sync_dirty_bitmap().
>>>
>>> It started out as a VGA optimization (also used by live migration) and
>>> even today it's mostly used by the VGA drivers - albeit a weak one.
>>>
>>> I wish there were stronger VGA optimizations implemented, copying the
>>> dirty bitmap is not a particularly performant solution. (although it's
>>> certainly better than full emulation) Graphics performance is one of the
>>> more painful aspects of KVM usability today.
>>>
>> We have to maintain a dirty bitmap because we don't have a paravirtual
>> graphics driver. IOW, someone needs to write an Xorg driver.
>>
>> Ideally, we could just implement a Linux framebuffer device, right?
>>
> No, you'd want to interact with DRM.
>

Using DRM doesn't help very much. You still need an X driver and most
of the operations you care about (video rendering, window movement, etc)
are not operations that need to go through DRM.

3D graphics virtualization is extremely difficult in the non-passthrough
case. It really requires hardware support that isn't widely available
today (outside a few NVIDIA chipsets).

>> Xorg framebuffer driver doesn't implement any of the optimizations that the
>> Linux framebuffer supports and the Xorg driver does not provide use the
>> kernel's interfaces for providing update regions.
>>
>> Of course, we need to pull in X into the kernel to fix this, right?
>>
> FYI, this part of X has already been pulled into the kernel, it's called DRM.
> If then it's being expanded.
>

It doesn't provide the things we need to a good user experience. You
need things like an absolute input device, host driven display resize,
RGBA hardware cursors. None of these go through DRI and it's those
things that really provide the graphics user experience.

>> Any sufficiently complicated piece of software is going to interact with a
>> lot of other projects. The solution is not to pull it all into one massive
>> repository. It's to build relationships and to find ways to efficiently
>> work with the various communities.
>>
> That's my whole point with this thread: the kernel side of KVM and qemu, but
> all practical purposes should not be two 'separate communities'. They should
> be one and the same thing.
>

I don't know why you keep saying this. The people who are in these
"separate communities" keep claiming that they don't feel this way.

I'm not just saying this to be argumentative. Many of the people in the
community have thought this same thing, and tried it themselves, and
we've all come to the same conclusion.

It's certainly possible that we just missed the obvious thing to do but
we'll never know that unless someone shows us.

> The thing is, writing up a DRM connector to a guest Linux OS could be done in
> no time. It could be deployed to users in no time as well, with the proper
> development model.
>

If this is true, please demonstrate it. Prove your point with patches
and I'll happily turn around and do whatever I can to help out.

> That after years and years of waiting proper GX support is _still_ not
> implemented in KVM is really telling of the efficiency of development based on
> such disjoint 'communities'. Maybe put up a committee as well to increase
> efficiency? ;-)
>

Nah, instead we can just have a few hundred mail thread on the list.
Otherwise we'd have to write patches and do other kinds of productive work.

Regards,

Anthony Liguori

> Ingo
>