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

[Avi Kivity]

On 03/10/2010 06:36 PM, Cam Macdonell wrote:
> On Wed, Mar 10, 2010 at 2:21 AM, Avi Kivity<address@hidden>  wrote:
>    
> > On 03/09/2010 08:34 PM, Cam Macdonell wrote:
> >      
> > > On Tue, Mar 9, 2010 at 10:28 AM, Avi Kivity<address@hidden>    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.
> >
> >      
> Do you mean how they synchronize without interrupts?  One project I've
> been contacted about uses the shared region directly for
> synchronization for simulations running in different VMs that share
> data in the memory region.  In my tests spinlocks in the shared region
> work between guests.
>    

I see.

> If we want to keep the serverless implementation, do we need to
> support shm_open with -chardev somehow? Something like -chardev
> shm,name=foo.  Right now my qdev implementation just passes the name
> to the -device option and opens it.
>    

I think using the file name is fine.

> > Another thing comes to mind - a shared memory ID, in case a guest has
> > multiple cards.
> >      
> Sure, a number that can be passed on the command-line and stored in a register?
>    

Yes.  NICs use the MAC address and storage uses the disk serial number, 
this is the same thing for shared memory.

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