Re: [dpdk-dev] [PATCH] doc: add knowing issue for i40e VF performance

["Mcnamara, John" <john.mcnamara@intel.com>]

> -----Original Message-----
> From: Zhang, Qi Z
> Sent: Sunday, July 2, 2017 1:07 PM
> To: Mcnamara, John <john.mcnamara@intel.com>; Wu, Jingjing
> <jingjing.wu@intel.com>
> Cc: Zhang, Helin <helin.zhang@intel.com>; dev@dpdk.org; Zhang, Qi Z
> <qi.z.zhang@intel.com>
> Subject: [PATCH] doc: add knowing issue for i40e VF performance
> 
> VF performance is limited by some kernel PCI setting.
> Update the document to explain the knowing issue and work around solution.
> 
> Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>
> ---
>  doc/guides/nics/i40e.rst | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/doc/guides/nics/i40e.rst b/doc/guides/nics/i40e.rst index
> 4d3c7ca..557d83d 100644
> --- a/doc/guides/nics/i40e.rst
> +++ b/doc/guides/nics/i40e.rst
> @@ -447,3 +447,27 @@ It means if APP has set the max bandwidth for that
> TC, it comes to no  effect.
>  It's suggested to set the strict priority mode for a TC that is latency
> sensitive but no consuming much bandwidth.
> +
> +VF performance is impacted by PCI extended tag setting
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +To reach maximum NIC performance. PCI extended tag is required to be
> enabled.
> +DPDK I40E PF drvier will set this feature during initialization, but
> +Kernel PF driver does not. So when running traffic on a VF which is
> +managed by kernel PF driver, we saw significent NIC performance
> +downgrade (for 64 bytes packet, there is about 25% linerate downgrade
> +for 25G device and about 35% for 40G device).
> +
> +Solution:
> +
> +For kernel version >= 4.11, kernel's PCI driver will enale extended tag
> +if it detects that device support extended tag. So by default, this is
> not an issue.
> +When extended tag is be disabled by occasionally, to re-enable it, see
> below.
> +
> +For kernel version < 4.11, use setpci command to enable PCI extended
> +flag
> +1) get current value of PCI configure register setpci -s <XX:XX.X> a8.w
> +2) set bit 8
> +value = value | 0x100
> +3) set PCI configure register with new value.
> +setpci -s <XX:XX.X> a8.w=<value>

To use a numbered list in the doc you should do it like this:

#. Get the current value of the PCI configure register::

      setpci -s <XX:XX.X> a8.w

#. Set bit 8::

      value = value | 0x100

#. Set the PCI configure register with new value::

      setpci -s <XX:XX.X> a8.w=<value>


John