Date:	Thu, 18 Apr 2013 15:51:06 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v2 7/7] xen-block: implement indirect descriptors

On 18/04/13 14:21, Konrad Rzeszutek Wilk wrote:
> On Wed, Apr 17, 2013 at 08:19:01PM +0200, Roger Pau Monne wrote:
>> Indirect descriptors introduce a new block operation
>> (BLKIF_OP_INDIRECT) that passes grant references instead of segments
>> in the request. This grant references are filled with arrays of
>> blkif_request_segment_aligned, this way we can send more segments in a
>> request.
>>
>> The proposed implementation sets the maximum number of indirect grefs
>> (frames filled with blkif_request_segment_aligned) to 256 in the
>> backend and 32 in the frontend. The value in the frontend has been
>> chosen experimentally, and the backend value has been set to a sane
>> value that allows expanding the maximum number of indirect descriptors
>> in the frontend if needed.
>>
>> The migration code has changed from the previous implementation, in
>> which we simply remapped the segments on the shared ring. Now the
>> maximum number of segments allowed in a request can change depending
>> on the backend, so we have to requeue all the requests in the ring and
>> in the queue and split the bios in them if they are bigger than the
>> new maximum number of segments.
>>
>> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>> Cc: xen-devel@...ts.xen.org
>> ---
>> Changes since v1:
>>  * Added padding to make the indirect request 64bit aligned.
>>  * Added some BUGs.
>>  * Added comments.
>>  * Fixed number of indirect pages in blkif_get_x86_{32/64}_req.
>>  * Added description about the indirect operation in blkif.h
> 
> Good. I only have three comments - and if you want to handle them later
> as seperate patches that is OK too.
> 
> They are:
> 
>> --- a/drivers/block/xen-blkback/xenbus.c
>> +++ b/drivers/block/xen-blkback/xenbus.c
>> @@ -107,6 +107,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>>  	struct xen_blkif *blkif;
>>  	int i;
>>  
>> +	BUILD_BUG_ON(MAX_INDIRECT_PAGES > BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST);
>> +
>>  	blkif = kmem_cache_zalloc(xen_blkif_cachep, GFP_KERNEL);
>>  	if (!blkif)
>>  		return ERR_PTR(-ENOMEM);
>> @@ -710,6 +712,12 @@ again:
>>  		goto abort;
>>  	}
>>  
>> +	err = xenbus_printf(xbt, dev->nodename, "feature-max-indirect-segments", "%u",
>> +	                    MAX_INDIRECT_SEGMENTS);
>> +	if (err)
>> +		xenbus_dev_fatal(dev, err, "writing %s/feature-max-indirect-segments",
>> +				 dev->nodename);
> 
> Not really fatal. We can continue on and just not use that feature.

Fixed, changed the fatal to pr_alert.

> 
> .. snip..
>> @@ -1483,6 +1799,13 @@ static void blkfront_connect(struct blkfront_info *info)
>>  	else
>>  		info->feature_persistent = persistent;
>>  
>> +	err = blkfront_setup_indirect(info);
>> +	if (err) {
>> +		xenbus_dev_fatal(info->xbdev, err, "setup_indirect at %s",
>> +				 info->xbdev->otherend);
>> +		return;
> 
> I thought we decided not to make it fatal?

If blkfront_setup_indirect returns an error, it doesn't mean it hasn't
been able to fetch the xenstore node, it means we are not able to
allocate necessary structures for blkfront to function properly (amongst
them the scatter-gather list), so it is fatal.

> 
>> +	}
>> +
>>  	err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
>>  	if (err) {
>>  		xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
>> diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
>> index ffd4652..7901e58 100644
>> --- a/include/xen/interface/io/blkif.h
>> +++ b/include/xen/interface/io/blkif.h
>> @@ -103,12 +103,41 @@ typedef uint64_t blkif_sector_t;
>>  #define BLKIF_OP_DISCARD           5
>>  
>>  /*
>> + * Recognized if "feature-max-indirect-segments" in present in the backend
>> + * xenbus info. The "feature-max-indirect-segments" node contains the maximum
>> + * number of segments allowed by the backend. If the node is present,
>                                                ^-'per request'.
> 
>> + * the frontend might use blkif_request_indirect in order to issue requests
>                                                    ^ structs
>> + * with more than BLKIF_MAX_SEGMENTS_PER_REQUEST. The maximum number
>                                                    ^(11)
> 
>> + * of indirect segments is fixed by the backend, but the frontend can issue
>> + * requests with any number of indirect segments as long as it's inferior
>                                                                     ^^- less 
>> + * than the number provided by the backend. The indirect_grefs field in
>> + * blkif_request_indirect should be filled by the frontend with the
>> + * grant references of the pages that are holding the indirect segments.
>> + * This pages are filled with an array of blkif_request_segment_aligned
>> + * that hold the information about the segments.
> 
> You should probably explain where the counter for the amount of indirect_grefs
> that are going to be used is computed.

Done :)

>> + *
>> + * If a backend does not recognize BLKIF_OP_INDIRECT, it should *not*
>> + * create the "feature-max-indirect-segments" node!
>> + */
>> +#define BLKIF_OP_INDIRECT          6

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