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Re: [Qemu-devel] [dpdk-dev] dpdk/vpp and cross-version migration for vho

From: Thomas Monjalon
Subject: Re: [Qemu-devel] [dpdk-dev] dpdk/vpp and cross-version migration for vhost
Date: Mon, 28 Nov 2016 23:18:53 +0100
User-agent: KMail/4.14.10 (Linux/4.5.4-1-ARCH; KDE/4.14.11; x86_64; ; )

2016-11-28 16:28, Maxime Coquelin:
> On 11/24/2016 04:24 PM, Kavanagh, Mark B wrote:
> > DPDK v16.04 added support for vHost User TSO; as such, by default,
> > TSO is advertised to guest devices as an available feature during
> > feature negotiation with QEMU.
> > However, while the vHost user backend sets up the majority of the
> > mbuf fields that are required for TSO, there is still a reliance
> > on the associated DPDK application (i.e. in this case OvS-DPDK)
> > to set the remaining flags and/or offsets.
> > Since OvS-DPDK doesn't currently provide that functionality, it is
> > necessary to explicitly disable TSO; otherwise, undefined behaviour
> > will ensue.
> Thanks Mark for the clarification.
> In this case, maybe we could add a DPDK build option to disable Vhost's
> TSO support, that would be selected for OVS packages?

Why do you prefer a build-time option rather than the run-time config
with rte_vhost_feature_disable()? Because we need to lock the features?

Reminder: build-time configuration options are forbidden in DPDK for
such usage. It would prevent other applications from using the feature
in a given distribution, just because it is not implemented in OVS.

> Does that sound reasonable?

Maybe I'm missing something but I feel it is more reasonnable to implement
the missing code in OVS.
If something is missing in DPDK, do not hesitate to request or add more
helper functions.

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