Re: blkback feature announcement

[Bob Liu <bob.liu@oracle.com>]

On 12/07/2015 08:42 PM, Roger Pau Monné 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().

Bob.

>> Or in more general terms: Shouldn't it be well defined at what time
>> a frontend can rely on certain nodes to be available for inspection?
>> And in doing so, I'd expect the determination to be done such that
>> widest flexibility is provided towards the actual implementation, i.e.
>> nodes should be written as early as possible. (Of course this applies
>> to other frontend/backend pairs too.)
> 
> I agree. Regarding blkback the nodes about persistent grants, indirect
> descriptors and the ring page order should be written in
> xen_blkbk_probe, while the specific information about this virtual disk
> (sectors, sector size...) should be written before switching to the
> connected state (ie: after hotplug scripts have run).
> 
> Roger.
>