Re: [Xen-devel] [PATCH v10 05/31] tools/libx{l, c}: add back channel to libxc

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Feb 22, 2016 at 10:52:09AM +0800, Wen Congyang wrote:
> In COLO mode, both VMs are running, and are considered in sync if the
> visible network traffic is identical.  After some time, they fall out of
> sync.
> 
> At this point, the two VMs have definitely diverged.  Lets call the
> primary dirty bitmap set A, while the secondary dirty bitmap set B.
> 
> Sets A and B are different.
> 
> Under normal migration, the page data for set A will be sent from the
> primary to the secondary.
> 
> However, the set difference B - A (the one in B but not in A, lets
> call this C) is out-of-date on the secondary (with respect to the
> primary) and will not be sent by the primary (to secondary), as it
> was not memory dirtied by the primary. The secondary needs C page data
> to reconstruct an exact copy of the primary at the checkpoint.
> 
> The secondary cannot calculate C as it doesn't know A.  Instead, the
> secondary must send B to the primary, at which point the primary
> calculates the union of A and B (lets call this D) which is all the
> pages dirtied by both the primary and the secondary, and sends all page
> data covered by D.
> 
> In the general case, D is a superset of both A and B.  Without the
> backchannel dirty bitmap, a COLO checkpoint can't reconstruct a valid
> copy of the primary.
> 
> We transfer the dirty bitmap on libxc side, so we need to introduce back
> channel to libxc.
> 
> Note: it is different from the paper. We change the original design to
> the current one, according to our following concerns:
> 1. The original design needs extra memory on Secondary host. When there's
>    multiple backups on one host, the memory cost is high.
> 2. The memory cache code will be another 1k+, it will make the review
>    more time consuming.
> 
> Note: the back channel will be used in the patch
>  libxc/restore: send dirty pfn list to primary when checkpoint under COLO
> to send dirty pfn list from secondary to primary. The patch is posted in
> another series.

I think you need to update the commit message a bit. I take the liberty
of writing one for you:

This patch merely adds new parameters to various prototypes and
functions. The new parameters are used in later patch called
"libxc/restore: send dirty pfn list to primary when checkpoint under
COLO".

I would normally ask the submitter to merge patch like this to the
actual patch that uses the parameter. It you can do that, that would be
great. But this patch series is already quite large, if this involves a
lot of conflict resolution on your side, just leave it as-is.

Either way, please confirm your intention before doing actual work.

Bottom line: patch like this needs to compile and not introduce any
regression. I think that's the case here.

Wei.

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