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

[Avi Kivity <avi@redhat.com>]

On 03/10/2010 04:04 PM, Arnd Bergmann wrote:
> 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.
>    

That would be much slower.  The current scheme allows for an 
ioeventfd/irqfd short circuit which allows one guest to interrupt 
another without involving their qemus at all.

-- 
error compiling committee.c: too many arguments to function