Re: [Xen-devel] [PATCH 3/3] Add shadow VRAM

Keir Fraser
Wed, 15 Mar 2006 23:06:39 -0000

On 15 Mar 2006, at 21:57, Donald D. Dugger wrote:

> Add a shadow VRAM to track changes to the real VRAM.  When the guest
> OS was given write access to the VRAM the device model tracked all
> VRAM changes by updating the entire screen on every output loop,
> causing significant overhead (a CPU bound loop in a guest slows down
> by about 35%) and significant mouse latency (VNC uses the same data
> path for mouse events and video updates).  With the shadow VRAM only
> modified pages need to be updated and the comparison of the shadow
> VRAM to the real VRAM only adds ~4% overhead while eliminating the
> mouse latencies.

Checking for SSE2 support on the build machine isn''t good enough, since
we may install on a totally different machine (distro binary packages,
for example). Can you make it a run-time decision? Shouldn''t add
significant overhead?

  -- Keir