RE: recvfrom/recvmsg performance and CONFIG_HARDENED_USERCOPY

[David Laight]

From: Eric Dumazet
> Sent: 06 December 2019 14:22
...
> Real question is : Do you actually need to use recvmsg() instead of recvfrom() ?
> If recvmsg() provides additional cmsg, this is not surprising it is more expensive.

Except I'm not passing in a buffer for it.
The reason I'm looking at recvmsg is that I'd like to use recvmmsg it order to
read out more than one message from a socket without doing an extra poll().
Note that I don't expect there to be a second message most of the time and
almost never a third one.

Although I think that will only ever 'win' if recvmmsg() called vfs_poll() to find
if there was more data to read before doing any of the copy_from_user() etc.

> recvmsg() also uses an indirect call, so CONFIG_RETPOLINE=y is probably hurting.

I don't have CONFIG_RETPOLINE enabled, the compiler I'm using is too old.
(gcc 4.7.3).

	David

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