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