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

[Avi Kivity]

On 03/09/2010 05:27 PM, Cam Macdonell wrote:
> > >   Registers are used
> > > for synchronization between guests sharing the same memory object when
> > > interrupts are supported (this requires using the shared memory server).
> > >
> > >        
> > How does the driver detect whether interrupts are supported or not?
> >      
> At the moment, the VM ID is set to -1 if interrupts aren't supported,
> but that may not be the clearest way to do things.  With UIO is there
> a way to detect if the interrupt pin is on?
>    

I suggest not designing the device to uio.  Make it a good 
guest-independent device, and if uio doesn't fit it, change it.

Why not support interrupts unconditionally?  Is the device useful 
without interrupts?

> > > The Doorbell register is 16-bits, but is treated as two 8-bit values.  The
> > > upper 8-bits are used for the destination VM ID.  The lower 8-bits are the
> > > value which will be written to the destination VM and what the guest
> > > status
> > > register will be set to when the interrupt is trigger is the destination
> > > guest.
> > >
> > >        
> > What happens when two interrupts are sent back-to-back to the same guest?
> >   Will the first status value be lost?
> >      
> Right now, it would be.  I believe that eventfd has a counting
> semaphore option, that could prevent loss of status (but limits what
> the status could be).
>    

It only counts the number of interrupts (and kvm will coalesce them anyway).

> My understanding of uio_pci interrupt handling
> is fairly new, but we could have the uio driver store the interrupt
> statuses to avoid losing them.
>    

There's nowhere to store them if we use ioeventfd/irqfd.  I think it's 
both easier and more efficient to leave this to the application (to 
store into shared memory).

--

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