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

[Paul Brook <paul@codesourcery.com>]

> 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.

Paul