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