From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v3 COLOPre 19/26] libxc/migration: Specification update for DIRTY_BITMAP records Date: Wed, 1 Jul 2015 11:39:24 +0100 Message-ID: <5593C35C.7000401@citrix.com> References: <1435213552-10556-1-git-send-email-yanghy@cn.fujitsu.com> <1435213552-10556-20-git-send-email-yanghy@cn.fujitsu.com> <1435659898.21469.79.camel@citrix.com> <55935959.3050606@cn.fujitsu.com> <5593BE0E.3000003@citrix.com> <1435746457.21469.242.camel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1435746457.21469.242.camel@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Ian Campbell Cc: wei.liu2@citrix.com, wency@cn.fujitsu.com, guijianfeng@cn.fujitsu.com, yunhong.jiang@intel.com, eddie.dong@intel.com, xen-devel@lists.xen.org, rshriram@cs.ubc.ca, Yang Hongyang , ian.jackson@eu.citrix.com List-Id: xen-devel@lists.xenproject.org On 01/07/15 11:27, Ian Campbell wrote: > On Wed, 2015-07-01 at 11:16 +0100, Andrew Cooper wrote: >> On 01/07/15 04:07, Yang Hongyang wrote: >>> >>> On 06/30/2015 06:24 PM, Ian Campbell wrote: >>>> On Thu, 2015-06-25 at 14:25 +0800, Yang Hongyang wrote: >>>>> Used by secondary to send it's dirty bitmap to primary under COLO. >>>> This is the backchannel, right? >>> Right. >>> >>>> It seems to me that this ought to be described more clearly as a >>>> separate stream in the opposite direction, rather than looking like just >>>> another record in the forward channel. >>> Agreed, I'm not sure if having this back channel record is eligible, >>> Andy, thoughts? >>> >>>> Does the back channel not also need some sort of negotiation phase where >>>> we check both ends are compatible (i.e. like the forward channel's >>>> header). This might be easier than with the forward channel since you >>>> might assert that the versions must match exactly for COLO to be >>>> possible, that might not be true of some potential future user of the >>>> backchannel though. >>> The negotiation record for COLO is introduced in the following patch >>> on libxl side. But that might be diffrent form what you said here, we >>> don't have a version check currently, if the 2 side doesn't match, for >>> example one has colo feature enabled and the other end do not, the >>> migration will simply fail. >> I do think that each backchannel level needs some kind of initial >> negotiation to confirm everything is set up and working, but I think the >> backchannel should also match the spec for its level, and all contained >> in the single spec document. > In the same spec, sure. It's the presenting it as just another record > mixed in with all the others which I think is a problem. Ah I see. Yes - this would better be avoided. > > At the very least every record should be tagged as either forward, > backward or bidirectional to indicate who can produce and who should > consume it. > > Even better would br if we can convince ourselves there should be no > bidirectional fields, in which case I'd be further inclined to say that > the record space should be explicitly separate. i.e. the backchannel > should be a separate chapter in the doc and the records. I think it would be unwise to rule out the possibility of bidirectional records. In the case that we get to a position of wanting/needing them, we absolutely don't want a bidirectional record to have different id in the forwards and backwards direction. ~Andrew > >> So for both the libxc and libxl backchannels, I would have thought >> something along these lines to be sensible: >> >> Forwards sends a LIBX{C,L}_BACKCHANNEL_INIT, and waits to find a >> LIBX{C,L}_BACKCHANNEL_REPLY on the backchannel. After that, processing >> continues as normal, with records arriving on the backchannel when >> appropriate. > Yes, for init time something like this sounds sensible. > > Ian. >