Re: [dpdk-dev] [PATCH] vhost: make iotlb cache name unique among multi processes

["Van Haaren, Harry" <harry.van.haaren@intel.com>]

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Ye Xiaolong
> Sent: Tuesday, March 10, 2020 11:32 AM
> To: Itsuro Oda <oda@valinux.co.jp>
> Cc: dev@dpdk.org; maxime.coquelin@redhat.com; Wang, Zhihong
> <zhihong.wang@intel.com>; stable@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] vhost: make iotlb cache name unique among
> multi processes
> 
> On 03/10, Itsuro Oda wrote:
> >Currently, iotlb cache name is comprised of vid and virtqueue
> >index. For example, "iotlb_cache_0_0". Because vid is assigned
> >per process, iotlb cache name is not unique among multi processes.
> >For example a secondary process uses a vhost
> >(ex. eth_vhost0,iface=/tmp/sock0) and another secondary process
> >uses a vhost (ex. eth_vhost1,iface=/tmp/sock1), iotlb cache
> >name of both vhost ("iotlb_cache_0_0") are same and as a result
> >iotlb cache is broken.
> >
> >This patch makes iotlb cache name unique among milti processes
> >by using the interface name not vid to comprise iotlb cache name.
> >Since the length of interface name is variable, this patch uses
> >hash value calculated by the interface name.
> >
> >Fixes: d012d1f293f4 (vhost: add IOTLB helper functions)
> >Cc: stable@dpdk.org
> >
> >Signed-off-by: Itsuro Oda <oda@valinux.co.jp>
> >---
> > lib/librte_vhost/iotlb.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> >diff --git a/lib/librte_vhost/iotlb.c b/lib/librte_vhost/iotlb.c
> >index bc1758528..0992c145b 100644
> >--- a/lib/librte_vhost/iotlb.c
> >+++ b/lib/librte_vhost/iotlb.c
> >@@ -6,6 +6,7 @@
> > #include <numaif.h>
> > #endif
> >
> >+#include <rte_jhash.h>
> > #include <rte_tailq.h>
> >
> > #include "iotlb.h"
> >@@ -288,6 +289,7 @@ vhost_user_iotlb_init(struct virtio_net *dev, int
> vq_index)
> > 	char pool_name[RTE_MEMPOOL_NAMESIZE];
> > 	struct vhost_virtqueue *vq = dev->virtqueue[vq_index];
> > 	int socket = 0;
> >+	uint32_t val;
> >
> > 	if (vq->iotlb_pool) {
> > 		/*
> >@@ -308,8 +310,10 @@ vhost_user_iotlb_init(struct virtio_net *dev, int
> vq_index)
> > 	TAILQ_INIT(&vq->iotlb_list);
> > 	TAILQ_INIT(&vq->iotlb_pending_list);
> >
> >-	snprintf(pool_name, sizeof(pool_name), "iotlb_cache_%d_%d",
> >-			dev->vid, vq_index);
> >+	val = rte_jhash(dev->ifname, strlen(dev->ifname), 0);
> >+	snprintf(pool_name, sizeof(pool_name), "iotlb_cache_%08x_%d",
> >+			val, vq_index);
> >+	VHOST_LOG_CONFIG(DEBUG, "IOTLB cache name: %s\n", pool_name);

Although very unlikely, what would happen if there is a hash-collision?

For example imagine two different names hash to the same "val", from
my understanding they will now use the same IOTLB but should not share one.

<snip>