Re: Fwd: [Xen-devel] Inter-domain shared memory facility in Xen?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

A couple of my longterm personal projects involve interdomain shared memory 
too.

One of these is a Xen version of the IBM zSeries DCSS (which is basically a 
big lump of shared memory, which can optionally be used as a disk).  The 
other is XenFS which presents a filesystem interface.

The end goal for either of those would include the ability to share userspace 
application memory between domains using just an mmap() call, but I've not 
got that working yet ;-)

Cheers,
Mark

On Thursday 01 November 2007, David Stone wrote:
> Thanks guys, I'm clear now.
>
> On Nov 1, 2007 10:16 AM, Derek Murray <Derek.Murray@xxxxxxxxxxxx> wrote:
> > David Stone wrote:
> > > Thanks for your help on this.  So based on your responses I just want
> > > to confirm my understanding is correct...is it true that:
> > >
> > > 1. It is currently possible to establish shared memory between Dom0
> > > and DomU, but
> > > 2. It is currently not possible to establish shared memory between two
> > > DomUs in an released Xen versions, nor the current xen-unstable branch
> > > nor any other branch on http://xenbits.xensource.org.  There are
> > > projects that address this that you mentioned though.
> >
> > It *is* possible to establish shared memory between two DomUs, using the
> > grant table mechanism. At present, it is only possible to grant access
> > to memory from a VM kernel, but it is possible to map that granted
> > memory into either the kernel (using the kernel wrappers to the
> > necessary hypercalls) or user-space (using gntdev, the user-space
> > granted memory driver).
> >
> > Up to and including the current stable release (3.1.1), it is only
> > possible for domains that have I/O memory permissions (typically only
> > Dom0) to map granted pages. Therefore, it is necessary to give some
> > dummy permissions to any domain that you want to allow to map grants.
> > However, the issue that led to this restriction has been fixed in the
> > current version of xen-unstable.
> >
> > As I understand it, the other projects that were mentioned provide a
> > socket-like communication mechanism for Xen, with the advantage that
> > they do not (necessarily) use the networking stack. I presume that you
> > are looking for something simpler, such as creating a shared data
> > structure between two or more domains. In that case, the mechanisms that
> >  exist in Xen today and that I have described above are sufficient for
> > your needs.
> >
> > Regards,
> >
> > Derek Murray.
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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