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

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.


Derek Murray.

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