[kernel/f21] Add patch to fix loop in VDSO (rhbz 1178975)

[Josh Boyer]

commit 9b4c496b180a73d5f9be065691942f6e9a1df36a
Author: Josh Boyer <jwboyer at fedoraproject.org>
Date:   Mon Jan 12 08:17:01 2015 -0500

    Add patch to fix loop in VDSO (rhbz 1178975)

 kernel.spec                                 |    9 +++++
 x86-vdso-Use-asm-volatile-in-__getcpu.patch |   48 +++++++++++++++++++++++++++
 2 files changed, 57 insertions(+), 0 deletions(-)
---
diff --git a/kernel.spec b/kernel.spec
index b5a0818..77ef8fc 100644
--- a/kernel.spec
+++ b/kernel.spec
@@ -651,6 +651,9 @@ Patch26122: batman-adv-Calculate-extra-tail-size-based-on-queued.patch
 #CVE-2014-9529 rhbz 1179813 1179853
 Patch26124: KEYS-close-race-between-key-lookup-and-freeing.patch
 
+#rhbz 1178975
+Patch26125: x86-vdso-Use-asm-volatile-in-__getcpu.patch
+
 # git clone ssh://git.fedorahosted.org/git/kernel-arm64.git, git diff master...devel
 Patch30000: kernel-arm64.patch
 
@@ -1411,6 +1414,9 @@ ApplyPatch batman-adv-Calculate-extra-tail-size-based-on-queued.patch
 #CVE-2014-9529 rhbz 1179813 1179853
 ApplyPatch KEYS-close-race-between-key-lookup-and-freeing.patch
 
+#rhbz 1178975
+ApplyPatch x86-vdso-Use-asm-volatile-in-__getcpu.patch
+
 %if 0%{?aarch64patches}
 ApplyPatch kernel-arm64.patch
 %ifnarch aarch64 # this is stupid, but i want to notice before secondary koji does.
@@ -2285,6 +2291,9 @@ fi
 #                                    ||----w |
 #                                    ||     ||
 %changelog
+* Mon Jan 12 2015 Josh Boyer <jwboyer at fedoraproject.org>
+- Add patch to fix loop in VDSO (rhbz 1178975)
+
 * Thu Jan 08 2015 Justin M. Forbes <jforbes at fedoraproject.org> - 3.17.8-300
 - Linux v3.17.8
 
diff --git a/x86-vdso-Use-asm-volatile-in-__getcpu.patch b/x86-vdso-Use-asm-volatile-in-__getcpu.patch
new file mode 100644
index 0000000..bcf5a15
--- /dev/null
+++ b/x86-vdso-Use-asm-volatile-in-__getcpu.patch
@@ -0,0 +1,48 @@
+From 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba Mon Sep 17 00:00:00 2001
+From: Andy Lutomirski <luto at amacapital.net>
+Date: Sun, 21 Dec 2014 08:57:46 -0800
+Subject: x86, vdso: Use asm volatile in __getcpu
+
+In Linux 3.18 and below, GCC hoists the lsl instructions in the
+pvclock code all the way to the beginning of __vdso_clock_gettime,
+slowing the non-paravirt case significantly.  For unknown reasons,
+presumably related to the removal of a branch, the performance issue
+is gone as of
+
+e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu
+
+but I don't trust GCC enough to expect the problem to stay fixed.
+
+There should be no correctness issue, because the __getcpu calls in
+__vdso_vlock_gettime were never necessary in the first place.
+
+Note to stable maintainers: In 3.18 and below, depending on
+configuration, gcc 4.9.2 generates code like this:
+
+     9c3:       44 0f 03 e8             lsl    %ax,%r13d
+     9c7:       45 89 eb                mov    %r13d,%r11d
+     9ca:       0f 03 d8                lsl    %ax,%ebx
+
+This patch won't apply as is to any released kernel, but I'll send a
+trivial backported version if needed.
+
+Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support
+Cc: stable at vger.kernel.org # 3.8+
+Cc: Marcelo Tosatti <mtosatti at redhat.com>
+Acked-by: Paolo Bonzini <pbonzini at redhat.com>
+Signed-off-by: Andy Lutomirski <luto at amacapital.net>
+(Backported to 3.17.8 by Josh Boyer <jwboyer at fedoraproject.org>)
+
+diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
+index 2a46ca720afc..3db1fa5fdceb 100644
+--- a/arch/x86/include/asm/vsyscall.h
++++ b/arch/x86/include/asm/vsyscall.h
+@@ -34,7 +34,7 @@ static inline unsigned int __getcpu(void)
+ 		native_read_tscp(&p);
+ 	} else {
+ 		/* Load per CPU data from GDT */
+-		asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
++		asm volatile("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
+ 	}
+ 
+ 	return p;