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


> 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.

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

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