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

[Avi Kivity]

On 05/11/2010 04:10 PM, Anthony Liguori wrote:
> On 05/11/2010 02:59 AM, Avi Kivity wrote:
> > > (Replying again to list)
> > >
> > > What data structure would you use?  For a lockless ring queue, you 
> > > can only support a single producer and consumer.  To achieve 
> > > bidirectional communication in virtio, we always use two queues.
> >
> >
> > You don't have to use a lockless ring queue.  You can use locks 
> > (spinlocks without interrupt support, full mutexes with interrupts) 
> > and any data structure you like.  Say a hash table + LRU for a shared 
> > cache.
>
> Yeah, the mailslot enables this.
>
> I think the question boils down to whether we can support transparent 
> peer connections and disconnections.  I think that's important in 
> order to support transparent live migration.
>
> If you have two peers that are disconnected and then connect to each 
> other, there's simply no way to choose who's content gets preserved.  
> It's necessary to designate one peer as a master in order to break the 
> tie.

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.

> So this could simply involve an additional option to the shared memory 
> driver: role=master|peer.  If role=master, when a new shared memory 
> segment is mapped, the contents of the BAR ram is memcpy()'d to the 
> shared memory segment.  In either case, the contents of the shared 
> memory segment should be memcpy()'d to the BAR ram whenever the shared 
> memory segment is disconnected.

I don't understand why we need separate BAR ram and shared memory.  Have 
just shared memory, exposed by the BAR when connected.  When the PCI 
card is disconnected from shared memory, the BAR should discard writes 
and return all 1s for reads.

Having a temporary RAM area while disconnected doesn't serve a purpose 
(since it exists only for a short while) and increases the RAM load.

> I believe role=master should be default because I think a relationship 
> of master/slave is going to be much more common than peering.

What if you have N guests?  What if the master disconnects?

> > > If you're adding additional queues to support other levels of 
> > > communication, you can always use different areas of shared memory.
> >
> > You'll need O(n^2) shared memory areas (n=peer count), and it is a 
> > lot less flexible that real shared memory.  Consider using threading 
> > where the only communication among threads is a pipe (erlang?)
>
> I can't think of a use of multiple peers via shared memory today with 
> virtualization.  I know lots of master/slave uses of shared memory 
> though.  I agree that it's useful to support from an academic 
> perspective but I don't believe it's going to be the common use.

Large shared cache.  That use case even survives live migration if you 
use lockless algorithms.

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