Patch "powerpc: Avoid taking a data miss on every userspace instruction miss" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-avoid-taking-a-data-miss-on-every-userspace-instruction-miss.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Anton Blanchard <[email protected]>
Date: Mon, 3 Apr 2017 16:41:02 +1000
Subject: powerpc: Avoid taking a data miss on every userspace instruction miss

From: Anton Blanchard <[email protected]>

[ Upstream commit a7a9dcd882a67b68568868b988289fce5ffd8419 ]

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.

---
 arch/powerpc/mm/fault.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -294,7 +294,7 @@ int __kprobes do_page_fault(struct pt_re
 	 * 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))

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/powerpc-avoid-taking-a-data-miss-on-every-userspace-instruction-miss.patch