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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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