Re: [Xen-devel] blkback feature announcement

[Bob Liu <bob.liu@xxxxxxxxxx>]

On 12/08/2015 07:13 PM, Jan Beulich wrote:
>>>> On 08.12.15 at 12:06, <bob.liu@xxxxxxxxxx> wrote:
> 
>> On 12/08/2015 04:13 PM, Jan Beulich wrote:
>>>>>> On 08.12.15 at 02:08, <bob.liu@xxxxxxxxxx> wrote:
>>>> On 12/07/2015 08:42 PM, Roger Pau Monnà wrote:
>>>>> El 07/12/15 a les 13.00, Jan Beulich ha escrit:
>>>>>> Hello,
>>>>>>
>>>>>> is there a particular reason why "max-ring-page-order" gets written in
>>>>>> xen_blkbk_probe(), but e.g. "feature-max-indirect-segments" and
>>>>>> "feature-persistent" get written only in connect(), despite both having
>>>>>> constant values (and hence the node value effectively being known as
>>>>>> soon as the device exists)?
>>>>>
>>>>> No, AFAIK there's no specific reason.
>>>>>
>>>>
>>>> AFAIR, that's for the blkfront resume path.
>>>>
>>>> We need to get the "max-ring-page-order" in blkfront_resume() in advance, 
>>>> so 
>>
>>>> that we can know how many ring pages to be used before setup_blkring().
>>>
>>> I don't follow - the proposal is to have the backend announce the
>>> feature _earlier_, so how could frontend resume be affected?
>>>
>>
>> The frontend resume is like this:
>>
>> blkfront_resume()
>>      > blkif_free()
>>      > talk_to_blkback()
>>          > setup_blkring() etc.
>>
>> blkback_changed()
>>      > blkfront_connect()
>>
>>
>> Sometimes the "max-ring-page-order" of backend may have changed after the 
>> guest(frontend) migrated to a different machine,
>> the frontend must aware of this change so that have to get the new value of 
>> "max-ring-page-order" in blkfront_resume().
>>
>> But it would be too late if the backend announces the "max-ring-page-order" 
>> in connect(), this situation is like:
>>
>> blkfront_resume()
>>      > blkif_free()
>>      > talk_to_blkback()
>>       ^^^ Get a wrong "max-ring-page-order"
>>          > setup_blkring() etc. but using the wrong value!!
>>
>> blkback_changed()
>>      > blkfront_connect()
>>        ^^^ Then the connect() in backend will be called(after frontend enter 
>> XenbusStateConnected) and write the correct "max-ring-page-order", but it's 
>> too late.
> 
> Oh, you're arguing for why "max-ring-page-order" is written early,
> but the question really was why others aren't written early too.
> 

Oh, sorry for misunderstood your point.
Yes, I agree that others can be written early too.

-- 
Regards,
-Bob

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