Re: [dpdk-dev] [PATCH v1 1/2] doc: update guides for vhost async APIs

["Fu, Patrick" <patrick.fu@intel.com>]

Thanks for comments. v2 patch sent with all the changes suggested.

Thanks,

Patrick


> -----Original Message-----
> From: Xia, Chenbo <chenbo.xia@intel.com>
> Sent: Wednesday, July 22, 2020 7:21 PM
> To: Fu, Patrick <patrick.fu@intel.com>; dev@dpdk.org;
> maxime.coquelin@redhat.com
> Subject: RE: [PATCH v1 1/2] doc: update guides for vhost async APIs
> 
> Hi Patrick,
> 
> > -----Original Message-----
> > From: Fu, Patrick <patrick.fu@intel.com>
> > Sent: Wednesday, July 22, 2020 6:58 PM
> > To: dev@dpdk.org; maxime.coquelin@redhat.com; Xia, Chenbo
> > <chenbo.xia@intel.com>
> > Cc: Fu, Patrick <patrick.fu@intel.com>
> > Subject: [PATCH v1 1/2] doc: update guides for vhost async APIs
> >
> > From: Patrick Fu <patrick.fu@intel.com>
> >
> > Update vhost guides to document vhost async APIs
> >
> > Signed-off-by: Patrick Fu <patrick.fu@intel.com>
> > ---
> >  doc/guides/prog_guide/vhost_lib.rst | 86
> > ++++++++++++++++++++++++++---
> >  1 file changed, 77 insertions(+), 9 deletions(-)
> >
> > diff --git a/doc/guides/prog_guide/vhost_lib.rst
> > b/doc/guides/prog_guide/vhost_lib.rst
> > index db921f922..cce8b6ae7 100644
> > --- a/doc/guides/prog_guide/vhost_lib.rst
> > +++ b/doc/guides/prog_guide/vhost_lib.rst
> > @@ -147,6 +147,21 @@ The following is an overview of some key Vhost
> > API
> > functions:
> >
> >      It is disabled by default.
> >
> > +  - ``RTE_VHOST_USER_ASYNC_COPY``
> > +
> > +    Asynchronous data path will be enabled when this flag is set. Async
> data
> > +    path allows applications to register async copy devices (typically
> > +    hardware DMA channels) to the vhost queues. Vhost leverages the copy
> > +    device registered to offload CPU from memory copy operations. A
> > + set of
> 
> You mean 'free' CPU from memory copy?
> 
> > +    async data path APIs are defined for DPDK applications to make use of
> > +    the async capability. Only packets enqueued/dequeued by async APIs
> are
> > +    processed through the async data path.
> > +
> > +    Currently this feature is only implemented on split ring enqueue data
> > +    path.
> > +
> > +    It is disabled by default.
> > +
> >  * ``rte_vhost_driver_set_features(path, features)``
> >
> >    This function sets the feature bits the vhost-user driver supports.
> > The @@ -
> > 235,6 +250,59 @@ The following is an overview of some key Vhost API
> > functions:
> >
> >    Enable or disable zero copy feature of the vhost crypto backend.
> >
> > +* ``rte_vhost_async_channel_register(vid, queue_id, features, ops)``
> > +
> > +  Register a vhost queue with async copy device channel.
> > +  Following device ``features`` must be specified together with the
> > +  registration:
> > +
> > +  * ``async_inorder``
> > +
> > +    Async copy device can guarantee the ordering of copy completion
> > +    sequence. Copies are completed in the same order with that at
> > +    the submission time.
> > +
> > +    Currently, only ``async_inorder`` capable device is supported by vhost.
> > +
> > +  * ``async_threshold``
> > +
> > +    The copy length (in bytes) below which CPU copy will be used even if
> > +    applications call async vhost APIs to enqueue/dequeue data.
> > +
> > +    Typical value is 512~1024 depending on the async device capability.
> > +
> > +  Applications must provide following ``ops`` callbacks for vhost lib
> > + to  work with the async copye devices:
> 
> s/copye/copy
> 
> > +
> > +  * ``transfer_data(vid, queue_id, descs, opaque_data, count)``
> > +
> > +    vhost invokes this function to submit copy data to the async devices.
> > +    For non-async_inorder capable devices, ``opaque_data`` could be used
> > +    for identifying the completed packets.
> > +
> > +  * ``check_completed_copies(vid, queue_id, opaque_data,
> > + max_packets)``
> > +
> > +    vhost invokes this function to get the copy data completed by async
> > +    devices.
> > +
> > +* ``rte_vhost_async_channel_unregister(vid, queue_id)``
> > +
> > +  Unregister the async copy device from a vhost queue.
> 
> 'Copy device channel' may be more accurate?
> 
> > +
> > +* ``rte_vhost_submit_enqueue_burst(vid, queue_id, pkts, count)``
> > +
> > +  Submit an enqueue request to transmit ``count`` packets from host
> > + to guest  by async data path. Enqueue is not guaranteed to finish
> > + upon the return of  this API call.
> > +
> > +  Applications must not free the packets submitted for enqueue until
> > + the  packets are completed.
> > +
> > +* ``rte_vhost_poll_enqueue_completed(vid, queue_id, pkts, count)``
> > +
> > +  Poll enqueue completion status from async data path. Completed
> > + packets  are returned to applications through ``pkts``.
> > +
> >  Vhost-user Implementations
> >  --------------------------
> >
> > @@ -294,16 +362,16 @@ Guest memory requirement
> >
> >  * Memory pre-allocation
> >
> > -  For non-zerocopy, guest memory pre-allocation is not a must. This
> > can help
> > -  save of memory. If users really want the guest memory to be
> > pre-allocated
> > -  (e.g., for performance reason), we can add option ``-mem-prealloc``
> > when
> > -  starting QEMU. Or, we can lock all memory at vhost side which will
> > force
> > -  memory to be allocated when mmap at vhost side; option --mlockall
> > in
> > -  ovs-dpdk is an example in hand.
> > +  For non-zerocopy non-async data path, guest memory pre-allocation
> > + is not a  must. This can help save of memory. If users really want
> > + the guest memory  to be pre-allocated (e.g., for performance
> > + reason), we can add option  ``-mem-prealloc`` when starting QEMU.
> > + Or, we can lock all memory at vhost  side which will force memory to
> > + be allocated when mmap at vhost side;  option --mlockall in ovs-dpdk is
> an example in hand.
> >
> > -  For zerocopy, we force the VM memory to be pre-allocated at vhost
> > lib when
> > -  mapping the guest memory; and also we need to lock the memory to
> > prevent
> > -  pages being swapped out to disk.
> > +  For async data path or zerocopy, we force the VM memory to be
> 
> 'For async or zerocopy data path' may be better?
> 
> Thanks!
> Chenbo
> 
> > + pre-allocated  at vhost lib when mapping the guest memory; and also
> > + we need to lock the  memory to prevent pages being swapped out to disk.
> >
> >  * Memory sharing
> >
> > --
> > 2.18.4