[Xen-devel] Re: vram_dirty vs. shadow paging dirty tracking

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Ian Pratt wrote:
> > When thinking about multithreading the device model, it occurred to me
> > that it's a little odd that we're doing a memcmp to determine which
> > portions of the VRAM has changed.  Couldn't we just use dirty page
> > tracking in the shadow paging code?  That should significantly lower
> > the
> > overhead of this plus I believe the infrastructure is already mostly
> > there in the shadow2 code.
> >     
> Yep, its been in the roadmap doc for quite a while. However, the log
> dirty code isn't ideal for this. We'd need to extend it to enable it to
> be turned on for just a subset of the GFN range (we could use a xen
> rangeset for this).
>   
Okay, I was curious if the log dirty stuff could do ranges.  I guess not.

> Even so, I'm not super keen on the idea of tearing down and rebuilding
> 1024 PTE's up to 50 times a second. 
>
> A lower overhead solution would be to do scanning and resetting of the
> dirty bits on the PTEs (and a global tlb flush).
Right, this is the approach I was assuming.  There's really no use in 
tearing down the whole PTE (since you would have to take an extraneous 
read fault).
> In the general case
> this is tricky as the framebuffer could be mapped by multiple PTEs. In
> practice, I believe this doesn't happen for either Linux or Windows.
>   
I wouldn't think so, but showing my ignorance for a moment, does shadow2 
not provide a mechanism to lookup VA's given a GFN?  This lookup could 
be cheap if the structures are built during shadow page table construction.
Sounds like this is a good long term goal but I think I'll stick with 
the threading as an intermediate goal.
I've got a minor concern that threading isn't going to help us much when 
dom0 is UP since the VGA scanning won't happen while an MMIO/PIO request 
happens.  With an SMP dom0, you could potentially do all the VGA 
scanning on one processor ensuring that qemu-dm wasn't ever "busy" when 
a request occurs.  I'm slightly concerned though that having a thread 
that's as CPU hungry as the VGA scanning may increase context-switches 
during the MMIO/PIO handling which would actually hurt performance.
We'll see soon enough though.

Regards,

Anthony Liguori

> There's always a good fallback of just returning 'all dirty' if the
> heuristic is violated. Would be good to knock this up.
>
> Best,
> Ian
>   

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