diff options
authorJunjie Chen <>2018-03-14 12:24:16 -0400
committerFerruh Yigit <>2018-04-14 00:43:30 +0200
commite3075e969eff1d4746b58172a188a4dc12194ef6 (patch)
parentbd8f50a45d0fb55cf191c3f6022a1ceb80a0b712 (diff)
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 <> Reviewed-by: Jianfeng Tan <> Reviewed-by: Maxime Coquelin <>
2 files changed, 9 insertions, 1 deletions
diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index 04ab1ee..ef9a178 100644
--- 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.
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 da161a9..5e7f24c 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 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