fix i8k inline asm to avoid miscompilation with newer gcc

2 files changed, 92 insertions, 0 deletions

diff --git a/fix-i8k-inline-asm.patch b/fix-i8k-inline-asm.patch
new file mode 100644
index 00000000..87fadc01
--- /dev/null
+++ b/fix-i8k-inline-asm.patch
@@ -0,0 +1,84 @@
+commit 22d3243de86bc92d874abb7c5b185d5c47aba323
+Author: Jim Bos <jim876@xs4all.nl>
+Date:   Mon Nov 15 21:22:37 2010 +0100
+
+    Fix gcc 4.5.1 miscompiling drivers/char/i8k.c (again)
+    
+    The fix in commit 6b4e81db2552 ("i8k: Tell gcc that *regs gets
+    clobbered") to work around the gcc miscompiling i8k.c to add "+m
+    (*regs)" caused register pressure problems and a build failure.
+    
+    Changing the 'asm' statement to 'asm volatile' instead should prevent
+    that and works around the gcc bug as well, so we can remove the "+m".
+    
+    [ Background on the gcc bug: a memory clobber fails to mark the function
+      the asm resides in as non-pure (aka "__attribute__((const))"), so if
+      the function does nothing else that triggers the non-pure logic, gcc
+      will think that that function has no side effects at all. As a result,
+      callers will be mis-compiled.
+    
+      Adding the "+m" made gcc see that it's not a pure function, and so
+      does "asm volatile". The problem was never really the need to mark
+      "*regs" as changed, since the memory clobber did that part - the
+      problem was just a bug in the gcc "pure" function analysis  - Linus ]
+    
+    Signed-off-by: Jim Bos <jim876@xs4all.nl>
+    Acked-by: Jakub Jelinek <jakub@redhat.com>
+    Cc: Andi Kleen <andi@firstfloor.org>
+    Cc: Andreas Schwab <schwab@linux-m68k.org>
+    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+
+commit 6b4e81db2552bad04100e7d5ddeed7e848f53b48
+Author: Jim Bos <jim876@xs4all.nl>
+Date:   Sat Nov 13 12:13:53 2010 +0100
+
+    i8k: Tell gcc that *regs gets clobbered
+    
+    More recent GCC caused the i8k driver to stop working, on Slackware
+    compiler was upgraded from gcc-4.4.4 to gcc-4.5.1 after which it didn't
+    work anymore, meaning the driver didn't load or gave total nonsensical
+    output.
+    
+    As it turned out the asm(..) statement forgot to mention it modifies the
+    *regs variable.
+    
+    Credits to Andi Kleen and Andreas Schwab for providing the fix.
+    
+    Signed-off-by: Jim Bos <jim876@xs4all.nl>
+    Cc: Andi Kleen <andi@firstfloor.org>
+    Cc: Andreas Schwab <schwab@linux-m68k.org>
+    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+
+---
+diff --git a/drivers/char/i8k.c b/drivers/char/i8k.c
+index 3bc0eef..d72433f 100644
+--- a/drivers/char/i8k.c
++++ b/drivers/char/i8k.c
+@@ -120,7 +120,7 @@ static int i8k_smm(struct smm_regs *regs)
+ 	int eax = regs->eax;
+ 
+ #if defined(CONFIG_X86_64)
+-	asm("pushq %%rax\n\t"
++	asm volatile("pushq %%rax\n\t"
+ 		"movl 0(%%rax),%%edx\n\t"
+ 		"pushq %%rdx\n\t"
+ 		"movl 4(%%rax),%%ebx\n\t"
+@@ -146,7 +146,7 @@ static int i8k_smm(struct smm_regs *regs)
+ 		:    "a"(regs)
+ 		:    "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");
+ #else
+-	asm("pushl %%eax\n\t"
++	asm volatile("pushl %%eax\n\t"
+ 	    "movl 0(%%eax),%%edx\n\t"
+ 	    "push %%edx\n\t"
+ 	    "movl 4(%%eax),%%ebx\n\t"
+@@ -167,7 +167,8 @@ static int i8k_smm(struct smm_regs *regs)
+ 	    "movl %%edx,0(%%eax)\n\t"
+ 	    "lahf\n\t"
+ 	    "shrl $8,%%eax\n\t"
+-	    "andl $1,%%eax\n":"=a"(rc)
++	    "andl $1,%%eax\n"
++	    :"=a"(rc)
+ 	    :    "a"(regs)
+ 	    :    "%ebx", "%ecx", "%edx", "%esi", "%edi", "memory");
+ #endif
diff --git a/kernel.spec b/kernel.spec
index adcef1b7..8b08076e 100644
--- a/kernel.spec
+++ b/kernel.spec
@@ -728,6 +728,8 @@ Patch12305: xhci_hcd-suspend-resume.patch
 
 Patch12307: tty-restore-tty_ldisc_wait_idle.patch
 
+Patch12308: fix-i8k-inline-asm.patch
+
 %endif
 
 BuildRoot: %{_tmppath}/kernel-%{KVERREL}-root
@@ -1350,6 +1352,8 @@ ApplyPatch xhci_hcd-suspend-resume.patch
 
 ApplyPatch tty-restore-tty_ldisc_wait_idle.patch
 
+ApplyPatch fix-i8k-inline-asm.patch
+
 # END OF PATCH APPLICATIONS
 
 %endif
@@ -1963,6 +1967,10 @@ fi
 #                 ||     ||
 
 %changelog
+* Tue Nov 23 2010 Kyle McMartin <kyle@redhat.com>
+- fix-i8k-inline-asm.patch: backport gcc miscompilation fix from git
+  [22d3243d, 6b4e81db] (rhbz#647677)
+
 * Mon Nov 22 2010 Kyle McMartin <kyle@redhat.com>
 - Add a debugging patch to help track down which tty is being
   poked by plymouth.