Re: [PATCH v5 4/5] Inter-VM shared memory PCI device

[Avi Kivity <avi@redhat.com>]

On 05/11/2010 05:17 PM, Cam Macdonell wrote:
>
>> The master is the shared memory area.  It's a completely separate entity
>> that is represented by the backing file (or shared memory server handing out
>> the fd to mmap).  It can exists independently of any guest.
>>      
> I think the master/peer idea would be necessary if we were sharing
> guest memory (sharing guest A's memory with guest B).  Then if the
> master (guest A) dies, perhaps something needs to happen to preserve
> the memory contents.

Definitely.  But we aren't...

>    But since we're sharing host memory, the
> applications in the guests can race to determine the master by
> grabbing a lock at offset 0 or by using lowest VM ID.
>
> Looking at it another way, it is the applications using shared memory
> that may or may not need a master, the Qemu processes don't need the
> concept of a master since the memory belongs to the host.
>    

Exactly.  Furthermore, even in a master/slave relationship, there will 
be different masters for different sub-areas, it would be a pity to 
expose all this in the hardware abstraction.  This way we have an 
external device, and PCI HBAs which connect to it - just like a 
multi-tailed SCSI disk.


-- 
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