doc: add driver limitation for vhost dequeue zero copy

In vhost-switch example, when binding nic to vfio-pci with iommu enabled,
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@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index 04ab1eeb493dccc7b8ac6d4a8e52e1b4d425f603..ef9a17877640f671d9d89722c1469e61812a8a7e 100644 (file)
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -83,6 +83,11 @@ 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 can not work when using vfio-pci with iommu mode currently, this
+      is because we don't setup iommu dma mapping for guest memory. If you have
+      to use vfio-pci driver, please insert vfio-pci kernel module in noiommu
+      mode.
+
   - ``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 da161a9a91ae93d020761fa38f9cc94f95f58d89..5e7f24c4e8bd75f940b176c1aba655c3825d78d2 100644 (file)
--- 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 driver with iommu enabled, 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