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.
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.
read/write is really just a much cleaner interface for anything that has
blocking semantics.
Regards,
Anthony Liguori
If you used a read/write interface, you could poll for any number of
processors and handle IO emulation in a single userspace thread
(which seems closer to how hardware really works anyway).
We can still do that by having the thread write an I/O request to
hardware service thread, and read back the response. However that
will not be too good for scheduling. For now the smp plan is to slap
a single lock on the qemu device model, and later fine-grain the
locking on individual devices as necessary.
Qemu's transition to aio will probably help in reducing the amount of
work done under lock.
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