powerpc: Avoid taking a data miss on every userspace instruction miss

Early on in do_page_fault() we call store_updates_sp(), regardless of
the type of exception. For an instruction miss this doesn't make
sense, because we only use this information to detect if a data miss
is the result of a stack expansion instruction or not.

Worse still, it results in a data miss within every userspace
instruction miss handler, because we try and load the very instruction
we are about to install a pte for!

A simple exec microbenchmark runs 6% faster on POWER8 with this fix:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
unsigned long left = atol(argv[1]);
char leftstr[16];

if (left-- == 0)
return 0;

sprintf(leftstr, "%ld", left);
execlp(argv[0], argv[0], leftstr, NULL);
perror("exec failed\n");

return 0;
}

Pass the number of iterations on the command line (eg 10000) and time
how long it takes to execute.

Signed-off-by: Anton Blanchard <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/mm/fault.c

···
 	 * can result in fault, which will cause a deadlock when called with
 	 * mmap_sem held
 	 */
-	if (user_mode(regs))
+	if (!is_exec && user_mode(regs))
 		store_update_sp = store_updates_sp(regs);
 
 	if (user_mode(regs))