Re: [PATCH 1/7] KVM: userspace interface

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
> Avi Kivity wrote:
> > Anthony Liguori wrote:
> > > ioctls are probably wrong here though.  Ideally, you would want to 
> > > be able to support an SMP guest.  This means you need to have two 
> > > virtual processors executing in kernel space.  If you use ioctls, it 
> > > forces you to have two separate threads in userspace.  This would be 
> > > hard for something like QEMU which is currently single threaded (and 
> > > not at all thread safe).
> >
> > Since we're using the Linux scheduler, we need a task per virtual cpu 
> > anyway, so a thread per vcpu is not a problem.
>
> You miss my point I think.  Using ioctls *requires* a thread per-vcpu 
> in userspace.  This is unnecessary since you could simply provide a 
> char-device based read/write interface.  You could then multiplex 
> events and poll.

Yes, ioctl()s require userspace threads, but that's okay, because 
they're free for us, since we need a kernel thread for each vcpu.

On the other hand, a single device model thread polling the vcpus is 
guaranteed to be on the wrong physical cpu for half of the time 
(assuming 2 cpus and 2 vcpus), requiring IPIs and suspending a vcpu in 
order to run.

> If for nothing else, you have to be able to run timers in userspace 
> and interrupt the kernel execution (to signal DMA completion for 
> instance).  Even in the UP case, this gets ugly quickly.

The timers aren't pretty (we use signals), yes.  But avoiding the extra 
thread is critical for performance IMO.

> read/write is really just a much cleaner interface for anything that 
> has blocking semantics.

Ah, but scheduling a vcpu doesn't just block, it consumes the physical cpu.

All other uses of read() yield the cpu apart from setup and copying of 
the data.


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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