[Qemu-devel] Re: [PATCH] Inter-VM shared memory PCI device

[Arnd Bergmann]

On Tuesday 09 March 2010, Cam Macdonell wrote:
> >
> > We could make the masking in RAM, not in registers, like virtio, which would
> > require no exits.  It would then be part of the application specific
> > protocol and out of scope of of this spec.
> >
> 
> This kind of implementation would be possible now since with UIO it's
> up to the application whether to mask interrupts or not and what
> interrupts mean.  We could leave the interrupt mask register for those
> who want that behaviour.  Arnd's idea would remove the need for the
> Doorbell and Mask, but we will always need at least one MMIO register
> to send whatever interrupts we do send.

You'd also have to be very careful if the notification is in RAM to
avoid races between one guest triggering an interrupt and another
guest clearing its interrupt mask.

A totally different option that avoids this whole problem would
be to separate the signalling from the shared memory, making the
PCI shared memory device a trivial device with a single memory BAR,
and using something a higher-level concept like a virtio based
serial line for the actual signalling.

        Arnd