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

[Avi Kivity]

On 03/08/2010 03:03 PM, Paul Brook wrote:
> > On 03/08/2010 12:53 AM, Paul Brook wrote:
> >      
> > > > Support an inter-vm shared memory device that maps a shared-memory
> > > > object as a PCI device in the guest.  This patch also supports
> > > > interrupts between guest by communicating over a unix domain socket.
> > > > This patch applies to the qemu-kvm repository.
> > > >          
> > > No. All new devices should be fully qdev based.
> > >
> > > I suspect you've also ignored a load of coherency issues, especially when
> > > not using KVM. As soon as you have shared memory in more than one host
> > > thread/process you have to worry about memory barriers.
> > >        
> > Shouldn't it be sufficient to require the guest to issue barriers (and
> > to ensure tcg honours the barriers, if someone wants this with tcg)?.
> >      
> In a cross environment that becomes extremely hairy.  For example the x86
> architecture effectively has an implicit write barrier before every store, and
> an implicit read barrier before every load.
>    

Ah yes.  For cross tcg environments you can map the memory using mmio 
callbacks instead of directly, and issue the appropriate barriers there.

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