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doc: add known issue for virtio TSO with clones

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Document the issue with Tso on shared packets.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
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Olivier Matz 2017-02-06 14:16:53 +01:00 committed by Thomas Monjalon
parent e736083b5d
commit 604a69493d
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doc/guides/rel_notes/known_issues.rst
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@ -666,3 +666,34 @@ uio pci generic module bind failed in X710/XL710/XXV710
**Driver/Module**:
Poll Mode Driver (PMD).
virtio tx_burst() function cannot do TSO on shared packets
----------------------------------------------------------
**Description**:
The standard TX function of virtio driver does not manage shared
packets properly when doing TSO. These packets should be read-only
but the driver modifies them.
When doing TSO, the virtio standard expects that the L4 checksum is
set to the pseudo header checksum in the packet data, which is
different than the DPDK API. The driver patches the L4 checksum to
conform to the virtio standard, but this solution is invalid when
dealing with shared packets (clones), because the packet data should
not be modified.
**Implication**:
In this situation, the shared data will be modified by the driver,
potentially causing race conditions with the other users of the mbuf
data.
**Resolution/Workaround**:
The workaround in the application is to ensure that the network
headers in the packet data are not shared.
**Affected Environment/Platform**:
Virtual machines running a virtio driver.
**Driver/Module**:
Poll Mode Driver (PMD).