[dpdk-dev] [PATCH v13 2/2] kni: support IOVA mode

[Ferruh Yigit]

On 11/15/2019 11:18 AM, vattunuru at marvell.com wrote:
> From: Vamsi Attunuru <vattunuru at marvell.com>
> 
> Current KNI implementation only operates in IOVA_PA mode
> patch adds required functionality to enable KNI in
> IOVA_VA mode.
> 
> KNI loopback mode tests will have performance impact in
> this mode due to IOVA to KVA address translations.
> However, In KNI real world use cases, the performace
> impact will be based on Linux kernel stack and scheduler
> latencies. Performance varies based on the KNI use case.
> If bus iommu scheme is IOVA_DC and KNI module is loaded,
> DPDK chooses IOVA as PA as existing behaviour.
> 
> During KNI creation, app's iova_mode details are passed to
> the KNI kernel module, accordingly kernel module translates
> PA/IOVA addresses to KVA and vice-versa.
> 
> Signed-off-by: Vamsi Attunuru <vattunuru at marvell.com>
> Signed-off-by: Kiran Kumar K <kirankumark at marvell.com>
> Suggested-by: Ferruh Yigit <ferruh.yigit at intel.com>
> ---
>  doc/guides/prog_guide/kernel_nic_interface.rst | 15 +++++++++++++++
>  doc/guides/rel_notes/release_19_11.rst         | 15 ++++++++++++++-
>  lib/librte_eal/linux/eal/eal.c                 | 23 ++++++++++++++++-------
>  lib/librte_kni/rte_kni.c                       |  6 ++++++
>  4 files changed, 51 insertions(+), 8 deletions(-)
> 
> diff --git a/doc/guides/prog_guide/kernel_nic_interface.rst b/doc/guides/prog_guide/kernel_nic_interface.rst
> index 2fd58e1..3bed18f 100644
> --- a/doc/guides/prog_guide/kernel_nic_interface.rst
> +++ b/doc/guides/prog_guide/kernel_nic_interface.rst
> @@ -300,6 +300,21 @@ The sk_buff is then freed and the mbuf sent in the tx_q FIFO.
>  The DPDK TX thread dequeues the mbuf and sends it to the PMD via ``rte_eth_tx_burst()``.
>  It then puts the mbuf back in the cache.
>  
> +IOVA = VA: Support
> +------------------
> +
> +KNI operates in IOVA_VA scheme when
> +
> +- LINUX_VERSION_CODE >= KERNEL_VERSION(4, 6, 0) and
> +- eal option `iova-mode=va` is passed or bus IOVA scheme in the DPDK is selected
> +  as RTE_IOVA_VA.
> +
> +KNI loopback mode tests will have performance impact in this mode due to IOVA to
> +KVA address translations. However, In KNI real world use cases, the performace
> +impact will be based on Linux kernel stack and scheduler latencies. Performance
> +varies based on the KNI use case. If bus iommu scheme is IOVA_DC and KNI module
> +is loaded, DPDK chooses IOVA as PA as existing behaviour.

I understand you are trying to neglect the affect but mentioning the Linux
kernel stack and scheduler latency etc is confusing I think, also I am for
mentioning the availability of the eal "--iova-mode=pa" option, what about
following like more simple message:

"
Due to IOVA to KVA address translations, based on the KNI use case there can be
a performance impact. For mitigation forcing IOVA to PA via eal "--iova-mode=pa"
option can be used.
"

IOVA_DC bus iommu scheme resulting IOVA as PA either can be another paragraph I
think, as a single sentences.