Re: [Xen-devel] Fast inter-VM signaling using monitor/mwait

[Michael Abd-El-Malek <mabdelmalek@xxxxxxx>]

On Apr 20, 2009, at 11:19 PM, Ian Pratt wrote:

> > The mwait instruction is privileged.  So I added a new hypercall that
> > wraps access to the mwait instruction.  Thus, my code has a Xen
> > component (the new hypercall) and a guest kernel component (code for
> > executing the hypercall and for turning off/on the timer interrupts
> > around the hypercall).  For this code to be merged into Xen, it would
> > need to add security checks and check whether the processor supports
> > such a feature.
> I seem to recall that some newer CPUs have an mwait instruction  
> accessible from ring3, using a different opcode -- you might want to  
> check this out.
Thanks for the pointer.  I'm not aware of these new instructions.  A  
quick Google search didn't turn up anything.  Can any of the Intel/AMD  
folks shed more light?
> How do you deal with atomicity of the monitor and mwait? i.e. how do  
> you stop the hypervisor pre-empting the VM and using monitor for its  
> own purposes or letting another guest use it?
The monitor and mwait instructions are _both_ executed in the  
hypervisor.  I should've been clearer: my new "mwait" hypercall  
executes both the monitor and mwait instructions.
> Have you thought about HVM guests as well as PV?
No, I haven't thought about HVM.  (I'm about to reply to Kevin TIan's  
response and hopefully we can figure this out.)
> Best,
> Ian
Thanks for the feedback!
Mike

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel