Re: [PATCH v3 COLOPre 16/26] tools/libx{l, c}: add back channel to libxc

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-07-01 at 10:38 +0800, Yang Hongyang wrote:
> 
> On 06/30/2015 06:10 PM, Ian Campbell wrote:
> > On Thu, 2015-06-25 at 14:25 +0800, Yang Hongyang wrote:
> >> We need to send secondary's dirty page pfns back to primary.
> >
> > In v2 Ian asked (<21888.2988.774072.32946@mariner.uk.xensource.com>):
> >
> >          In the pdf
> >             http://www.socc2013.org/home/program/a3-dong.pdf?attredirects=0
> >          linked from the wiki page
> >             http://wiki.xen.org/wiki/COLO_-_Coarse_Grain_Lock_Stepping
> >          it says that the secondary keeps a copy of the original contents of
> >          its dirty pages.  So I don't understand why you need to send the dirty
> >          bitmap to the primary.
> >
> > Which I don't see an answer for in my archive. Have I missed (or
> > misplaced) the answer?
> 
> Sorry, seems that I misplaced the answer to:
> [PATCH v2 COLOPre 09/13] tools/libxl: Update libxl_save_msgs_gen.pl to support 
> return data from xl to xc
> 
>    > Thanks for this.  I would have some comments on the details, but first
>    > I want to properly understand your use case.  So while I'm the author
>    > and maintainer of this save helper, I won't review this in detail just
>    > yet.  I'm following the thread about what this is for...
> 
>      We need to send secondary's dirty page pfn back to primary. Primary will
>      then send pages that are both dirtied on primary/secondary to secondary.
>      in this way the secondary's memory will be consistent with primary.
> 
>      As we disscussed in [PATCH v2 COLOPre 04/13] tools/libxc: export xc_bitops.h
>      If we move this operation to libxc layer, this patch could be dropped.

This doesn't seem to be a response to Ian's question which I quoted
above.

The crux of the question is that the design contained in those links
does not appear to require a back channel, because it does not require a
dirty bitmap to go from secondary to primary. Asserting a need to do so
does not answer the question.

Ian.