Re: [PATCH RFC v2 6/9] xen/arm: Add handling write fault for dirty-page tracing

[Jaeyong Yoo <jaeyong.yoo@samsung.com>]

> -----Original Message-----
> From: Ian Campbell [mailto:Ian.Campbell@citrix.com]
> Sent: Thursday, July 04, 2013 5:47 PM
> To: Jaeyong Yoo
> Cc: xen-devel@lists.xen.org; 'Elena Pyatunina'
> Subject: Re: [Xen-devel] [PATCH RFC v2 6/9] xen/arm: Add handling write
> fault for dirty-page tracing
> 
> On Thu, 2013-07-04 at 10:02 +0900, Jaeyong Yoo wrote:
> 
> > > I had been considering the option of reusing one of the bits
> > > available for software use in the p2m entries to track the dirty
> > > status and walking the p2m when the toolstack asked for the current
> bitmap instead.
> > >
> > > I think this would have less memory overhead and also would allow us
> > > to be lock free on the fault handling path (by using suitable atomic
> > > exchanges on the p2m entries), as well as avoiding memory allocations
> on that path.
> > >
> > > I think we'd want to establish a linear map of the current guest p2m
> > > for these purposes so we could also avoid all the map_domain_page
> > > stuff in the fault path. I think there is enough virtual address
> > > space left for that even on 32-bit, it's less critical on 64-bit
> > > anyway since we are about to have a direct map of RAM available to us.
> >
> > I think it is a good idea to have an ever-mapping linear dirty-bit map
> > for guest p2m.
> 
> By Linear map I meant slotting the p2m into the hypervisor's own page
> tables (I believe the relevant p2m and pte bits in the entries do not
> overlap, so this is possible), such that the p2m table entries are
> available at known virtual addresses and you can locate the L1, L2 and
> L3 entries corresponding to a given guest physical address with just
> arithmetic, i.e. something like:
> http://www.technovelty.org/linux/virtual-linear-page-table.html
> 

Oh it looks interesting. And we can try this. Thanks for the good tip :)

> As opposed to an always mapped version of the dirty bitmap, which I think
> we can sync only when the tools ask.

Got it.

Jaeyong