netdev - Re: [PATCH bpf] uapi/bpf: Remove text about bpf_redirect

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Date:   Tue, 18 Feb 2020 10:33:57 +0000
From:   Quentin Monnet <quentin@...valent.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        daniel@...earbox.net, ast@...com
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH bpf] uapi/bpf: Remove text about bpf_redirect_map() giving
 higher performance

2020-02-18 08:46 UTC+0100 ~ Toke Høiland-Jørgensen <toke@...hat.com>
> The performance of bpf_redirect() is now roughly the same as that of
> bpf_redirect_map(). However, David Ahern pointed out that the header file
> has not been updated to reflect this, and still says that a significant
> performance increase is possible when using bpf_redirect_map(). Remove this
> text from the bpf_redirect_map() description, and reword the description in
> bpf_redirect() slightly.
> 
> Fixes: 1d233886dd90 ("xdp: Use bulking for non-map XDP_REDIRECT and consolidate code paths")
> Suggested-by: David Ahern <dsahern@...il.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>   include/uapi/linux/bpf.h | 12 +++---------
>   1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f1d74a2bd234..7a526d917fb3 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1045,9 +1045,9 @@ union bpf_attr {
>    * 		supports redirection to the egress interface, and accepts no
>    * 		flag at all.
>    *
> - * 		The same effect can be attained with the more generic
> - * 		**bpf_redirect_map**\ (), which requires specific maps to be
> - * 		used but offers better performance.
> + * 		The same effect can also be attained with the more generic
> + * 		**bpf_redirect_map**\ (), which uses a BPF map to store the
> + * 		redirect target instead of providing it directly to the helper.
>    * 	Return
>    * 		For XDP, the helper returns **XDP_REDIRECT** on success or
>    * 		**XDP_ABORTED** on error. For other program types, the values
> @@ -1610,12 +1610,6 @@ union bpf_attr {
>    * 		one of the XDP program return codes up to XDP_TX, as chosen by
>    * 		the caller. Any higher bits in the *flags* argument must be
>    * 		unset.
> - *
> - * 		When used to redirect packets to net devices, this helper
> - * 		provides a high performance increase over **bpf_redirect**\ ().
> - * 		This is due to various implementation details of the underlying
> - * 		mechanisms, one of which is the fact that **bpf_redirect_map**\
> - * 		() tries to send packet as a "bulk" to the device.
>    * 	Return
>    * 		**XDP_REDIRECT** on success, or **XDP_ABORTED** on error.
>    *
> 

We could maybe leave something like “See also bpf_redirect()" in the 
description of "bpf_redirect_map()"?

Either way,
Reviewed-by: Quentin Monnet <quentin@...valent.com>

Thanks,
Quentin