Re: [PATCH net-next] net: skb: free up one bit in tx_flags

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 20 Feb 2025 22:58:20 -0500 Willem de Bruijn wrote:
> The linked series wants to add skb tx completion timestamps.
> That needs a bit in skb_shared_info.tx_flags, but all are in use.
> 
> A per-skb bit is only needed for features that are configured on a
> per packet basis. Per socket features can be read from sk->sk_tsflags.
> 
> Per packet tsflags can be set in sendmsg using cmsg, but only those in
> SOF_TIMESTAMPING_TX_RECORD_MASK.
> 
> Per packet tsflags can also be set without cmsg by sandwiching a
> send inbetween two setsockopts:
> 
>     val |= SOF_TIMESTAMPING_$FEATURE;
>     setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val));
>     write(fd, buf, sz);
>     val &= ~SOF_TIMESTAMPING_$FEATURE;
>     setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &val, sizeof(val));
> 
> Changing a datapath test from skb_shinfo(skb)->tx_flags to
> skb->sk->sk_tsflags can change behavior in that case, as the tx_flags
> is written before the second setsockopt updates sk_tsflags.
> 
> Therefore, only bits can be reclaimed that cannot be set by cmsg and
> are also highly unlikely to be used to target individual packets
> otherwise.
> 
> Free up the bit currently used for SKBTX_HW_TSTAMP_USE_CYCLES. This
> selects between clock and free running counter source for HW TX
> timestamps. It is probable that all packets of the same socket will
> always use the same source.

This needs a respin, presumably because Jason's series reached net-next
already.
-- 
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