Re: [Qemu-devel] add 'monitor' and 'mwait' instruction (update)

[Paul Brook <paul@codesourcery.com>]

> > Problem is, at the moment I've no idea, how we could achieve this memory
> > monitoring in a safe and simple way in user space.
>
> I'm trying to read up on monitor and mwait. Apparently mwait puts the
> processor in low-power wait mode, waiting for a memory write in some
> select area defined by monitor; and as I am new to this I'm not sure if
> I have understood all sources from where such a memory write can come
> from while the processor is asleep. One source, I suppose, is from other
> processors in an SMP setup? Another source may be DMA? Does this mean
> that it is safe to emulate wmait as hlt if neiter SMP or DMA is used?
> (Qemu hardware doesn't support DMA, right?)

qemu hardware does support DMA, but I don't think this matters.
By my reading DMA writes don't need to wake mwait.
The exact wording is "store operation", which I'd expect to mean execution of 
a store instruction (by a different CPU). So for UP systems mwait can be 
implemented the same as HLT, except that it raises an exception if state 
hasn't been initialized by monitor.

Paul