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

[Chen, Junjie J]

> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
> Sent: Tuesday, March 6, 2018 4:57 PM
> To: Chen, Junjie J <junjie.j.chen at intel.com>; yliu at fridaylinux.org; Tan,
> Jianfeng <jianfeng.tan at intel.com>
> Cc: dev at dpdk.org
> Subject: Re: [PATCH v2] doc: add driver limitation for vhost dequeue zero copy
> 
> 
> 
> 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.
You are right, it works on noiommu mode. Updated patch accordingly. 
> 
> >     - ``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
> >