Linux mainline fork with MSM8998 patches | https://mainline.space | Currently supported devices:
OnePlus 5/5T, Xiaomi Mi 6, F(x)tec Pro¹ (2019 QX1000 model) & Sony Xperia XZ Premium (UNTESTED!)
7eab503b11
It is possible for one CPU to perform TX hashing (see netdev_pick_tx) between the 8 ENETC TX rings, and the TX hashing to select TX queue 1. At the same time, it is possible for the other CPU to already use TX ring 1 for XDP (either XDP_TX or XDP_REDIRECT). Since there is no mutual exclusion between XDP and the network stack, we run into an issue because the ENETC TX procedure is not reentrant. The obvious approach would be to just make XDP take the lock of the network stack's TX queue corresponding to the ring it's about to enqueue in. For XDP_REDIRECT, this is quite straightforward, a lock at the beginning and end of enetc_xdp_xmit() should do the trick. But for XDP_TX, it's a bit more complicated. For one, we do TX batching all by ourselves for frames with the XDP_TX verdict. This is something we would like to keep the way it is, for performance reasons. But batching means that the network stack's lock should be kept from the first enqueued XDP_TX frame and until we ring the doorbell. That is mostly fine, except for cases when in the same NAPI loop we have mixed XDP_TX and XDP_REDIRECT frames. So if enetc_xdp_xmit() gets called while we are holding the lock from the RX NAPI, then bam, deadlock. The naive answer could be 'just flush the XDP_TX frames first, then release the network stack's TX queue lock, then call xdp_do_flush_map()'. But even xdp_do_redirect() is capable of flushing the batched XDP_REDIRECT frames, so unless we unlock/relock the TX queue around xdp_do_redirect(), there simply isn't any clean way to protect XDP_TX from concurrent network stack .ndo_start_xmit() on another CPU. So we need to take a different approach, and that is to reserve two rings for the sole use of XDP. We leave TX rings 0..ndev->real_num_tx_queues-1 to be handled by the network stack, and we pick them from the end of the priv->tx_ring array. We make an effort to keep the mapping done by enetc_alloc_msix() which decides which CPU handles the TX completions of which TX ring in its NAPI poll. So the XDP TX ring of CPU 0 is handled by TX ring 6, and the XDP TX ring of CPU 1 is handled by TX ring 7. Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.