linux-kernel - [PATCH 4/5] powerpc/mm: Evaluate user_mode(regs) only once in do_page

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Date:   Wed, 19 Apr 2017 14:56:30 +0200 (CEST)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH 4/5] powerpc/mm: Evaluate user_mode(regs) only once in
 do_page_fault()

Analysis of the assembly code shows that when using user_mode(regs),
at least the 'andi.' is redone all the time, and also
the 'lwz ,132(r31)' most of the time. With the new form, the 'is_user'
is mapped to cr4, then all further use of is_user results in just
things like 'beq cr4,218 <do_page_fault+0x218>'

Without the patch:

  50:	81 1e 00 84 	lwz     r8,132(r30)
  54:	71 09 40 00 	andi.   r9,r8,16384
  58:	40 82 00 0c 	bne     64 <do_page_fault+0x64>

  84:	81 3e 00 84 	lwz     r9,132(r30)
  8c:	71 2a 40 00 	andi.   r10,r9,16384
  90:	41 a2 01 64 	beq     1f4 <do_page_fault+0x1f4>

  d4:	81 3e 00 84 	lwz     r9,132(r30)
  dc:	71 28 40 00 	andi.   r8,r9,16384
  e0:	41 82 02 08 	beq     2e8 <do_page_fault+0x2e8>

 108:	81 3e 00 84 	lwz     r9,132(r30)
 110:	71 28 40 00 	andi.   r8,r9,16384
 118:	41 82 02 28 	beq     340 <do_page_fault+0x340>

 1e4:	81 3e 00 84 	lwz     r9,132(r30)
 1e8:	71 2a 40 00 	andi.   r10,r9,16384
 1ec:	40 82 01 68 	bne     354 <do_page_fault+0x354>

 228:	81 3e 00 84 	lwz     r9,132(r30)
 22c:	71 28 40 00 	andi.   r8,r9,16384
 230:	41 82 ff c4 	beq     1f4 <do_page_fault+0x1f4>

 288:	71 2a 40 00 	andi.   r10,r9,16384
 294:	41 a2 fe 60 	beq     f4 <do_page_fault+0xf4>

 50c:	81 3e 00 84 	lwz     r9,132(r30)
 514:	71 2a 40 00 	andi.   r10,r9,16384
 518:	40 a2 fc e0 	bne     1f8 <do_page_fault+0x1f8>

 534:	81 3e 00 84 	lwz     r9,132(r30)
 53c:	71 2a 40 00 	andi.   r10,r9,16384
 540:	41 82 fc b8 	beq     1f8 <do_page_fault+0x1f8>

This patch creates a local var called 'is_user' which contains the
result of user_mode(regs)

With the patch:

  20:	81 03 00 84 	lwz     r8,132(r3)
  48:	55 09 97 fe 	rlwinm  r9,r8,18,31,31
  58:	2e 09 00 00 	cmpwi   cr4,r9,0
  5c:	40 92 00 0c 	bne     cr4,68 <do_page_fault+0x68>

  88:	41 b2 01 90 	beq     cr4,218 <do_page_fault+0x218>

  d4:	40 92 01 d0 	bne     cr4,2a4 <do_page_fault+0x2a4>

 120:	41 b2 00 f8 	beq     cr4,218 <do_page_fault+0x218>

 138:	41 b2 ff a0 	beq     cr4,d8 <do_page_fault+0xd8>

 1d4:	40 92 00 e0 	bne     cr4,2b4 <do_page_fault+0x2b4>

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 arch/powerpc/mm/fault.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index b56bf472db6d..8d1639eee3af 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -202,6 +202,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	int is_write = 0;
 	int trap = TRAP(regs);
  	int is_exec = trap == 0x400;
+	int is_user = user_mode(regs);
 	int fault;
 	int rc = 0;
 	unsigned int inst = 0;
@@ -244,7 +245,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * The kernel should never take an execute fault nor should it
 	 * take a page fault to a kernel address.
 	 */
-	if (!user_mode(regs) && (is_exec || (address >= TASK_SIZE))) {
+	if (!is_user && (is_exec || (address >= TASK_SIZE))) {
 		rc = SIGSEGV;
 		goto bail;
 	}
@@ -263,7 +264,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 		local_irq_enable();
 
 	if (faulthandler_disabled() || mm == NULL) {
-		if (!user_mode(regs)) {
+		if (!is_user) {
 			rc = SIGSEGV;
 			goto bail;
 		}
@@ -284,10 +285,10 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * can result in fault, which will cause a deadlock when called with
 	 * mmap_sem held
 	 */
-	if (is_write && user_mode(regs))
+	if (is_write && is_user)
 		__get_user(inst, (unsigned int __user *)regs->nip);
 
-	if (user_mode(regs))
+	if (is_user)
 		flags |= FAULT_FLAG_USER;
 
 	/* When running in the kernel we expect faults to occur only to
@@ -306,7 +307,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * thus avoiding the deadlock.
 	 */
 	if (!down_read_trylock(&mm->mmap_sem)) {
-		if (!user_mode(regs) && !search_exception_tables(regs->nip))
+		if (!is_user && !search_exception_tables(regs->nip))
 			goto bad_area_nosemaphore;
 
 retry:
@@ -506,7 +507,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 
 bad_area_nosemaphore:
 	/* User mode accesses cause a SIGSEGV */
-	if (user_mode(regs)) {
+	if (is_user) {
 		_exception(SIGSEGV, regs, code, address);
 		goto bail;
 	}
-- 
2.12.0