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

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

On Apr 21, 2009, at 5:01 AM, Tian, Kevin wrote:

> > From: Ian Pratt
> > Sent: 2009年4月21日 11:19
> >
> > > 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.
> >
> > 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?
> That's a true concern. To use monitor/mwait sanely, software is  
> required
> to not add voluntary context switch in between, however to ensure that
> atomicity at hypercall level, I'm not sure about overall efficiency  
> when
> multiple VMs are all active...
I'm executing the montior and mwait instructions together in the  
hypercall.  The hypercall also takes an argument specifying the old  
value of the memory location.  When the mwait instruction returns, the  
hypervisor can check and handle any interrupts.  I currently return a  
continuation so that the mwait hypercall is rexecuted at the end of  
handling interrupts.  I haven't really thought about what if the VM  
gets scheduled out.  These are the kinds of issues that I'd like to  
fix if the community wants to add this hypercall.  For my benchmarking  
purposes, I'm not worrying about this :)
> > Have you thought about HVM guests as well as PV?
> For HVM guest, both vmexit and vmentry clears any address range
> monitoring in effect and thus that won't work.
I imagine this would cause the mwait instruction to execute before a  
write occurs to the memory address?  If so, the guest OS can check  
this (by comparing the memory address's value to the previous saved  
value), and reexecute the mwait hypercall.  Users of mwait already  
have to check whether their terminating condition has occurred, since  
interrupts cause mwait to return.
Thanks for the feedback,
Mike
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