Re: [PATCH v2 COLOPre 04/13] tools/libxc: export xc_bitops.h

[Yang Hongyang <yanghy@cn.fujitsu.com>]

On 06/11/2015 06:55 PM, Ian Campbell wrote:
> On Thu, 2015-06-11 at 11:45 +0100, Andrew Cooper wrote:
>> On 11/06/15 09:41, Ian Campbell wrote:
>>> On Thu, 2015-06-11 at 10:07 +0800, Yang Hongyang wrote:
>>>> On 06/10/2015 11:20 PM, Ian Campbell wrote:
>>>>> On Mon, 2015-06-08 at 11:43 +0800, Yang Hongyang wrote:
>>>>>> When we are under COLO, we will send dirty page bitmap info from
>>>>>> secondary to primary at every checkpoint.
>>>>> ... and this is a _libxl_ operation? Is that the right layer here?
>>>> For the first question, Yes, this is done in the suspend callback on
>>>> restore side. We do this in libxl because currently we only added a
>>>> back channel on libxl side. There're no back channel in libxc.
>>>>
>>>> By considering this more, if we do this in libxc part, the code will be
>>>> less complex: we can drop the 4th & 9th patch of this series and also
>>>> get rid of the get_dirty_pfn() callback. instead we will add a patch to
>>>> add back channel in libxc.
>>> That sounds better to me, but lets see what Andrew thinks.
>>>
>>>> For the second question, I'm not sure, what's Andrew's opinion? which
>>>> is the right layer to do this operation, libxl or libxc?
>>
>> There are a number of bits of information which would be useful going in
>> "the backchannel".
>>
>> Some are definitely more appropriate at the libxc level, but others are
>> more appropriate at the libxl.
>>
>> If you recall from the hackathon, there was an Alibaba usecase where
>> they wanted a positive success/fail from the receiving side that the VM
>> has started up successfully before choosing between cleaning up or
>> continuing the VM on the sending side.  This would have to be a libxl
>> level backchannel.
>
> FWIW this particular case is currently an xl level backchannel, but I
> think your general point stands.

So are you both agree that we should add a backchannel to libxc, move this
operation to libxc layer, what's other tools maintainers's opinion?

>
>> Whatever happens, backchannel wise, it should be a sensibly
>> type/length/chunk'd stream.  (I think there is a spec or two floating
>> around somewhere which might be a good start ;p)  There should probably
>> be a bit of active negotiation at the start of the backchannel to a)
>> confirm you have the correct backchannel and b) the backchannel is
>> actually functioning.
>>
>> The data on "the backchannel" is always going to be in reply to an
>> action taking place in the primary channel, but there are complications
>> in that the libxc bit is inherently a blocking model.  In terms of
>> coordination, I am leaning towards the view of it being easier and
>> cleaner for each level to maintain its own backchannel communication.
>> The libxc bits can expect to read some records out of the backchannel at
>> each checkpoint and take appropriate actions before starting the next
>> checkpoint.
>>
>> Thoughts?
>>
>> ~Andrew
>>
>
>
> .
>

-- 
Thanks,
Yang.