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

[Jamie Lokier <jamie@shareable.org>]

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.

Btw, x86 doesn't have any implicit barriers due to ordinary loads.
Only stores and atomics have implicit barriers, afaik.

-- Jamie