[dpdk-dev] [PATCH v4] doc: add driver limitation for vhost dequeue zero copy

[Maxime Coquelin]

On 03/14/2018 05:24 PM, Junjie Chen wrote:
> 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 at intel.com>

Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>

John, as this is a doc patch for Vhost, does it go through the doc tree 
or through Virtio one?

Thanks,
Maxime
> ---
> Changes in V4:
> - make words more accurate
> Changes in V3:
> - update limitation to iommu
> Changes in V2:
> - add doc in vhost lib
>   doc/guides/prog_guide/vhost_lib.rst | 5 +++++
>   doc/guides/sample_app_ug/vhost.rst  | 5 ++++-
>   2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
> index 18227b6..b49e36e 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.
> +
>     - ``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..f0bb169 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 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
>