RE: [Xen-devel] [PATCH]RFC: VGA accleration using shadow PTE D-bit to construct LFB dirty bitmap

["Ian Pratt" <Ian.Pratt@xxxxxxxxxxxx>]

> If there is one kernel-mode mapping of the LFB, though, I'd 
> expect it to be fairly rarely torn down; it would need the 
> shadows of all processes that were ever running when the 
> kernel touched the video RAM to be reaped.  So just marking 
> the page dirty when you make or tear down a vram mapping 
> should be better.

Ideally, we'd return 'all-dirty' for a missing shadow page just once,
and then 'all-clean' on subsequent scans until the PT page gets
reshadowed. 

I can imagine that if an OS'es screen blanker kicked in we might not see
any further writes to the LFB, and this could lead to the shadows
ultimately getting evicted, resulting in pessimal performance if we're
always returning 'all-dirty'.

There's no real need to do this on a page granularity -- if any of our
LFB-mapping shadow pages have been evicted we could evict them all. I'm
not sure this makes things any easier, though.

We can use missing shadows as an optimization: if we return a bit map
with 'everything clean' a few times in a row, we are probably better off
pro-actively unshadowing the page to avoid even doing the dirty bit
scanning.

Best,
Ian

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