Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect grant with 64KB pages

[Julien Grall <julien.grall@citrix.com>]

Hi Roger,

On 06/10/2015 10:39, Roger Pau Monné wrote:
> El 05/10/15 a les 19.05, Julien Grall ha escrit:
>> On 05/10/15 17:01, Roger Pau Monné wrote:
>>> El 11/09/15 a les 21.32, Julien Grall ha escrit:
>>>>   		ring_req->u.rw.nr_segments = num_grant;
>>>> +		if (unlikely(require_extra_req)) {
>>>> +			id2 = blkif_ring_get_request(info, req, &ring_req2);
>>>
>>> How can you guarantee that there's always going to be another free
>>> request? AFAICT blkif_queue_rq checks for RING_FULL, but you don't
>>> actually know if there's only one slot or more than one available.
>>
>> Because the depth of the queue is divided by 2 when the extra request is
>> used (see xlvbd_init_blk_queue).

I just noticed that I didn't mention this restriction in the commit 
message. I will do it in the next revision.

> I see, that's quite restrictive but I guess it's better than introducing
> a new ring macro in order to figure out if there are at least two free
> slots.

I actually didn't think about your suggestion. I choose to divide by two 
based on the assumption that the block framework will always try to send 
a request with the maximum data possible.

I don't know if this assumption is correct as I'm not fully aware how 
the block framework is working.

If it's valid, in the case of 64KB guest, the maximum size of a request 
would be 64KB when indirect segment is not supported. So we would end up 
with a lot of 64KB request which will require 2 ring request.

Regards,

-- 
Julien Grall