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

Ingo Molnar
2010-03-18T14:09:33Z

* Avi Kivity <avi@redhat.com> wrote:

> > That is not what i said. I said they are closely related, and where
> > technologies are closely related, project proximity turns into project
> > unification at a certain stage.
>
> I really don't see how. So what if both qemu and kvm implement an i8254?
> They can't share any code since the internal APIs are so different. [...]

I wouldnt jump to assumptions there. perf shares some facilities with the
kernel on the source code level - they can be built both in the kernel and in
user-space.

But my main thought wasnt even to actually share the implementation - but to
actually synchronize when a piece of device emulation moves into the kernel.
It is arguably bad for performance in most cases when Qemu handles a given
device - so all the common devices should be kernel accelerated.

The version and testing matrix would be simplified significantly as well: as
kernel and qemu goes hand in hand, they are always on the same version.

> [...] Even worse for the x86 emulator as qemu and kvm are fundamentally
> different.

So is it your argument that the difference and the duplication in x86
instruction emulation is a good thing? You said it some time ago that
the kvm x86 emulator was very messy and you wish it was cleaner.

While qemu's is indeed rather different (it's partly a translator/JIT), i'm
sure the decoder logic could be shared - and qemu has a slow-path
full-emulation fallback in any case, which is similar to what in-kernel
emulator does (IIRC ...).

That might have changed meanwhile.

Ingo