RE: [RFC][PATCH 01/22] x86 user stack frame reads: switch to explicit __get_user()

[David Laight]

From: Andy Lutomirski
> Sent: 29 March 2020 17:51
..
> My incliniation is to just get rid of the __get_user()-style APIs.
> There shouldn't be any __get_user() calls that can't be directly
> replaced by get_user(), and a single integer comparison is not that
> expensive.  On SMAP systems, the speedup of __get_user vs get_user is
> negligible.

On x86-64 (at least) __get_user() is inlined but get_user() isn't.
Since get_user() has to return two values, one will always be
a (usually) on-stack real memory location rather than a register.
For frequently use code paths this may be measurable.
I'm thinking of things like epoll_wait() writing out events.
(although that is a put_user() loop...)

It may be worth implementing get_user() as an inline
function that writes the result of access_ok() to a
'by reference' parameter and then returns the value
from an 'real' __get_user() function.
The compiler will then optimise away the memory reference.

	David

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