[dpdk-dev] [PATCH v2] doc: add driver limitation for vhost dequeue zero copy

[Junjie Chen]

In vhost-switch example, when binding nic to vfio-pci, dequeue zero
copy cannot work in VM2NIC mode due to no iommu dma mapping is setup
for guest memory currently.

Signed-off-by: Junjie Chen <junjie.j.chen at intel.com>
---
Changes in V2:
 - add doc in vhost lib

 doc/guides/prog_guide/vhost_lib.rst | 3 +++
 doc/guides/sample_app_ug/vhost.rst  | 5 ++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index 18227b6..bdf77d6 100644
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -83,6 +83,9 @@ The following is an overview of some key Vhost API functions:
       of those segments, thus the fewer the segments, the quicker we will get
       the mapping. NOTE: we may speed it by using tree searching in future.
 
+    * zero copy does not work when using vfio-pci driver currently, this is
+      because we don't setup iommu dma mapping for guest memory.
+
   - ``RTE_VHOST_USER_IOMMU_SUPPORT``
 
     IOMMU support will be enabled when this flag is set. It is disabled by
diff --git a/doc/guides/sample_app_ug/vhost.rst b/doc/guides/sample_app_ug/vhost.rst
index a4bdc6a..840c1fd 100644
--- a/doc/guides/sample_app_ug/vhost.rst
+++ b/doc/guides/sample_app_ug/vhost.rst
@@ -147,7 +147,10 @@ retries on an RX burst, it takes effect only when rx retry is enabled. The
 default value is 15.
 
 **--dequeue-zero-copy**
-Dequeue zero copy will be enabled when this option is given.
+Dequeue zero copy will be enabled when this option is given, it is worth to
+note that if NIC is binded to vfio-pci driver, dequeue zero copy cannot work
+at VM2NIC mode (vm2vm=0) due to currently we don't setup iommu dma mapping for
+guest memory.
 
 **--vlan-strip 0|1**
 VLAN strip option is removed, because different NICs have different behaviors
-- 
2.0.1