RE: [PATCH] m68k: Implement "current_stack_pointer"

[David Laight]

From: Geert Uytterhoeven
> Sent: 24 February 2022 08:59
...
> > +register unsigned long current_stack_pointer __asm__("sp");
> 
> I don't know what HARDENED_USERCOPY does, so I don't know if you need
> "usp" (see rdusp()) or "sp"?

HARDENED_USERCOPY significantly slows down some systems calls
(especially things like sendmsg()) by trying to run-time verify
that the kernel buffer doesn't overrun a stack frame or kmalloc()ed
buffer.

I've got measurable improvements by either using __copy_to/from_user()
(which skips the tests) or user_access_begin() and __get_user().

At the moment the code for reading a compat iovec[] is actually
faster than that for a native one!

	David

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