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

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

> >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.
> 
> Unshadowing & re-shadowing the all LFB pages are expensive.
> Performance would vary because the characteristic of graphic-intensive
> workload and the value of N -- the times of continuous 'all-clean'.
> I'm not sure this brings suffient benefit.

It's not that expensive, and it would be good to avoid the scanning
altogether for a screen that is updating very rarely (e.g. screen
blanker enabled). You could leave the shadows in place but write protect
the entries in the L2. However, since the shadows are liable to get
evicted in that scenario anyhow it may be an unnecessary complexity.

Ian

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