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

Maxime Coquelin maxime.coquelin at redhat.com
Tue Mar 6 09:57:07 CET 2018



On 02/27/2018 10:21 AM, Junjie Chen wrote:
> 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.
> +

I guess that it should work with vfio-pci in noiommu mode? Maybe worth
to clarify.

>     - ``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
> 


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