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doc: add known issue for i40e VF performance

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VF performance is limited by the kernel PCI extended tag setting.
Update the document to explain the known issue and the workaround.

Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>
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Qi Zhang 2017-07-18 05:52:14 -04:00 committed by Ferruh Yigit
parent 30ad2bdb42
commit 365b1b4bc6
1 changed files with 27 additions and 0 deletions
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doc/guides/nics/i40e.rst
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@ -447,3 +447,30 @@ 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 in the VF the PCI extended tag must be
enabled. The DPDK I40E PF driver will set this feature during initialization,
but the kernel PF driver does not. So when running traffic on a VF which is
managed by the kernel PF driver, a significant NIC performance downgrade has
been observed (for 64 byte packets, there is about 25% linerate downgrade for
a 25G device and about 35% for a 40G device).
For kernel version >= 4.11, the kernel's PCI driver will enable the extended
tag if it detects that the device supports it. So by default, this is not an
issue. For kernels <= 4.11 or when the PCI extended tag is disabled it can be
enabled using the steps below.
#. 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>