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

[Avi Kivity <avi@redhat.com>]

On 03/09/2010 08:34 PM, Cam Macdonell wrote:
> On Tue, Mar 9, 2010 at 10:28 AM, Avi Kivity<avi@redhat.com>  wrote:
>    
>> 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?
>>      
> Currently my patch works with or without the shared memory server.  If
> you give the parameter
>
> -ivshmem 256,foo
>
> then this will create (if necessary) and map /dev/shm/foo as the
> shared region without interrupt support.  Some users of shared memory
> are using it this way.
>
> Going forward we can require the shared memory server and always have
> interrupts enabled.
>    

Can you explain how they synchronize?  Polling?  Using the network?  
Using it as a shared cache?

If it's a reasonable use case it makes sense to keep it.

Another thing comes to mind - a shared memory ID, in case a guest has 
multiple cards.

-- 
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