From: Eamon Walsh <ewalsh@tycho.nsa.gov>
Subject: Re: [PATCH 2/2] xen-gntalloc: Userspace grant	allocation
	driver
Date: Wed, 08 Dec 2010 20:14:25 -0500
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To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Daniel De Graaf <dgdegra@tycho.nsa.gov>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>
List-Id: xen-devel@lists.xenproject.org

On 12/08/2010 11:47 AM, Ian Campbell wrote:
> On Fri, 2010-12-03 at 15:38 +0000, Daniel De Graaf wrote:
>> This allows a userspace application to allocate a shared page for
>> implementing inter-domain communication or device drivers. These
>> shared pages can be mapped using the gntdev device or by the kernel
>> in another domain.
> This seems like useful functionality but is it really necessary for it
> to be a separate driver to the existing gntdev driver? The broad high
> level semantics of ioctl+mmap seem pretty similar. It also has some
> similarities with the sort of device we will need in order to properly
> allocate memory which is safe to use as an argument to a hypercall.
>
> Do you have an example of a user of the driver?
>
> Thanks,
> Ian.
>

We are using gntalloc with the vchan library that Daniel posted for doing userspace ring buffers.

Another use of it that we are pursuing is for graphics.  Gntalloc would allow the X server to share one or more virtual framebuffers directly instead of using the xen-fbfront kernel driver.   This would allow multiple X sessions or screens to be forwarded from the same guest.  We are also investigating the possibility of applications rendering individual windows into their own buffers and then sharing them to a display server in another domain.


-- 

Eamon Walsh 
National Security Agency