Re: [PATCH v4 08/10] xen/blkback: get the number of hardware queues/rings from blkfront

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, Nov 02, 2015 at 12:21:44PM +0800, Bob Liu wrote:
> Backend advertises "multi-queue-max-queues" to front, then get the negotiated

s/then/so/
> number from "multi-queue-num-queues" wrote by blkfront.
s/wrote/written/

> 
> Signed-off-by: Bob Liu <bob.liu@oracle.com>
> ---
>  drivers/block/xen-blkback/blkback.c | 11 +++++++++++
>  drivers/block/xen-blkback/common.h  |  1 +
>  drivers/block/xen-blkback/xenbus.c  | 35 +++++++++++++++++++++++++++++------
>  3 files changed, 41 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index eaf7ec0..107cc4a 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -83,6 +83,11 @@ module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
>  MODULE_PARM_DESC(max_persistent_grants,
>                   "Maximum number of grants to map persistently");
>  
> +unsigned int xenblk_max_queues;

Do you want a default value?

> +module_param_named(max_queues, xenblk_max_queues, uint, 0644);
> +MODULE_PARM_DESC(max_queues,
> +		 "Maximum number of hardware queues per virtual disk");
> +
>  /*
>   * Maximum order of pages to be used for the shared ring between front and
>   * backend, 4KB page granularity is used.
> @@ -1478,6 +1483,12 @@ static int __init xen_blkif_init(void)
>  		xen_blkif_max_ring_order = XENBUS_MAX_RING_PAGE_ORDER;
>  	}
>  
> +	/* Allow as many queues as there are CPUs if user has not
> +	 * specified a value.

That should be part of the module_param actually.
> +	 */
> +	if (xenblk_max_queues == 0)
> +		xenblk_max_queues = num_online_cpus();
> +
>  	rc = xen_blkif_interface_init();
>  	if (rc)
>  		goto failed_init;
> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> index 4de1326..fb28b91 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -45,6 +45,7 @@
>  #include <xen/interface/io/protocols.h>
>  
>  extern unsigned int xen_blkif_max_ring_order;
> +extern unsigned int xenblk_max_queues;
>  /*
>   * This is the maximum number of segments that would be allowed in indirect
>   * requests. This value will also be passed to the frontend.
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index ac4b458..cafbadd 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -181,12 +181,6 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>  	INIT_LIST_HEAD(&blkif->persistent_purge_list);
>  	INIT_WORK(&blkif->persistent_purge_work, xen_blkbk_unmap_purged_grants);
>  
> -	blkif->nr_rings = 1;
> -	if (xen_blkif_alloc_rings(blkif)) {
> -		kmem_cache_free(xen_blkif_cachep, blkif);
> -		return ERR_PTR(-ENOMEM);
> -	}
> -
>  	return blkif;
>  }
>  
> @@ -606,6 +600,14 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
>  		goto fail;
>  	}
>  
> +	/* Multi-queue: write how many queues are supported by the backend. */
> +	err = xenbus_printf(XBT_NIL, dev->nodename,
> +			    "multi-queue-max-queues", "%u", xenblk_max_queues);
> +	if (err) {
> +		pr_warn("Error writing multi-queue-num-queues\n");
> +		goto fail;

Why fail? If we can't - then we can't and won't advertise this support.
We should still be able to work, right?

That is how the ''max-ring-page-order' does it below.
> +	}
> +
>  	/* setup back pointer */
>  	be->blkif->be = be;
>  
> @@ -997,6 +999,7 @@ static int connect_ring(struct backend_info *be)
>  	char *xspath;
>  	size_t xspathsize;
>  	const size_t xenstore_path_ext_size = 11; /* sufficient for "/queue-NNN" */
> +	unsigned int requested_num_queues = 0;
>  
>  	pr_debug("%s %s\n", __func__, dev->otherend);
>  
> @@ -1024,6 +1027,26 @@ static int connect_ring(struct backend_info *be)
>  	be->blkif->vbd.feature_gnt_persistent = pers_grants;
>  	be->blkif->vbd.overflow_max_grants = 0;
>  
> +	/*
> +	 * Read the number of hardware queues from frontend.
> +	 */
> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "multi-queue-num-queues", "%u", &requested_num_queues);

Please split this in two lines.

> +	if (err < 0) {
> +		requested_num_queues = 1;
> +	} else {
> +		if (requested_num_queues > xenblk_max_queues
> +		    || requested_num_queues == 0) {
> +			/* buggy or malicious guest */
> +			xenbus_dev_fatal(dev, err,
> +					"guest requested %u queues, exceeding the maximum of %u.",
> +					requested_num_queues, xenblk_max_queues);
> +			return -1;
> +		}
> +	}
> +	be->blkif->nr_rings = requested_num_queues;
> +	if (xen_blkif_alloc_rings(be->blkif))
> +		return -ENOMEM;
> +
>  	pr_info("nr_rings:%d protocol %d (%s) %s\n", be->blkif->nr_rings,
>  		 be->blkif->blk_protocol, protocol,
>  		 pers_grants ? "persistent grants" : "");
> -- 
> 1.8.3.1
>