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

[Avi Kivity]

On 03/18/2010 04:09 PM, Ingo Molnar wrote:
> * Avi Kivity<avi@xxxxxxxxxx>  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.
>    

So, you propose to allow running tools/kvm/ only on the kernel it was 
shipped with?

Otherwise the testing matrix isn't simplified.

> > [...] 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?

Of course it isn't a good thing, but it is unavoidable.  Qemu compiles 
code just-in-time to avoid interpretation overhead, while kvm emulates 
one instruction at a time.  No caching is possible, especially with 
ept/npt, since the guest is free to manipulate memory with no 
notification to the host.  Qemu also supports the full instruction set 
while kvm only implements what is necessary.  Qemu is a 
multi-source/multi-target translator while kvm's emulator is x86 specific.

> You said it some time ago that
> the kvm x86 emulator was very messy and you wish it was cleaner.
>    

It's still messy but is being cleaned up.

> 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.
>    

IIUC it only ever translates.

--
error compiling committee.c: too many arguments to function

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