Re: [Xen-devel] Mapping Data between Dom0 and DomU

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/01/2015 01:22, Christian Refvik wrote:
> Hello,
>
> My name is Christian and I am new to Xen development, and I've been 
> struggling a bit.  I'm trying to develop a device driver so that a Windows 
> 2012 Server VM  has a way to send about 10MB of data to a CentOS VM.  There 
> is no real device on the backend, I just need a way to constantly send a 
> large buffer between DomU and Dom0 using a zero copy solution.
>
> I wrote a Linux device driver (simple char device) in Dom0 which allocates 
> the memory and reserves the pages.  However, I'm not sure how to offer these 
> pages to the Windows VM.  I also have a Windows device driver that I've 
> installed in the Windows VM, but again I'm not sure how to connect the two 
> drivers.
>
> I've been browsing the QEMU source code, the Windows PV source code, the Xen 
> source code, and the "Definitive Guide to the Xen Hypervisor" for answers.  
> Unfortunately, I am a bit lost.  I'm hoping someone with some experience with 
> shared data communication between Dom0 and DomU can point me at some source 
> files to study in order to figure out how to implement what I'm trying to do. 
>  A tutorial, sample code, and/or general direction would be great.
>
> Thanks for your help,
> Christian

You will want to use grants, and in particular grant mappings, which is
the Xen interface for creating shared memory between VMs.

The basic premise is that domain A nominates some of its pages to be
grantable.  Domain A gives the grant references to domain B (usually
negotiated via xenstore), which allows domain B to make a mapping of
domain A's nominated pages.

A relevant bit of code to look at would be tools/libvchan/ , which as
far as I am aware does pretty much what you describe (but without the
windows support).

~Andrew

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