Re: [Qemu-devel] [PATCH -V3 09/32] virtio-9p: Implement P9_TWRITE/ Thread model in QEMU

[Paul Brook <paul@codesourcery.com>]

>>> 1) state machines with a thread pool to make blocking functions
>>> asynchronous (what we have today)
>>>
>>> 2) co-operative threading
>>>
>>> 3) pre-emptive threading

> > On a philosophical note, threads may be easier to model complex
> > hardware that includes a processor, for example our scsi card (and how
> > about using tcg as a jit to boost it :)
> 
> Yeah, it's hard to argue that script evaluation shouldn't be done in a
> thread.  But that doesn't prevent me from being very cautious about how
> and where we use threading :-)

I agree that making script execution asynchronous is a good thing, however I'm 
not convinced that (3) is the best way to do this. It's certainly the most 
flexible model, however it also places responsibility for locking/correctness 
on the device developer.

A more limited scheme (e.g. asynchronous callbacks) can actually be 
significant advantage. By forcing the developer to solve the problem in a 
particular way we significantly reduce the scope for race conditions, and 
hopefully restrict all locking concerns to common code/APIs.

Paul

[1] This issue may come from accidental misuse of terminology, but it's an 
important distinction.