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

[Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>]

At 13:11 +0100 on 30 Jul (1185801076), Ian Pratt wrote:
> 
> > 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. 

Yes, that's the behaviour you get if you mark the page dirty when you
clear the PTE that mapped it.  It will then get seen once as dirty and
not again util it's mapped again.  (Not-present and read-only PTEs won't
cause the page to be marked dirty because they can't cause it to have
changed.)

> 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.

True, and easily checked.

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, XenSource UK Limited
Registered office c/o EC2Y 5EB, UK; company number 05334508

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