Patch "s390/boot/decompression: disable floating point in decompressor" has been added to the 4.2-stable tree

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

s390/boot/decompression: disable floating point in decompressor

to the 4.2-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:
s390-boot-decompression-disable-floating-point-in-decompressor.patch
and it can be found in the queue-4.2 subdirectory.

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

From: Christian Borntraeger <[email protected]>
Date: Mon, 28 Sep 2015 22:47:42 +0200
Subject: s390/boot/decompression: disable floating point in decompressor

From: Christian Borntraeger <[email protected]>

commit adc0b7fbf6fe9967505c0254d9535ec7288186ae upstream.

my gcc 5.1 used an ldgr instruction with a register != 0,2,4,6 for spilling/filling into a floating point register in our decompressor.

This will cause an AFP-register data exception as the decompressor did not setup the additional floating point registers via cr0. That causes a program check loop that looked like a hang with one "Uncompressing Linux... " message (directly booted via kvm) or a loop of "Uncompressing Linux... " messages (when booted via zipl boot loader).

The offending code in my build was

   48e400:       e3 c0 af ff ff 71       lay     %r12,-1(%r10)
-->48e406:       b3 c1 00 1c             ldgr    %f1,%r12
   48e40a:       ec 6c 01 22 02 7f       clij    %r6,2,12,0x48e64e

but gcc could do spilling into an fpr at any function. We can simply disable floating point support at that early stage.

---
 arch/s390/boot/compressed/Makefile |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -10,7 +10,7 @@ targets += misc.o piggy.o sizes.h head.o
 
 KBUILD_CFLAGS := -m64 -D__KERNEL__ $(LINUX_INCLUDE) -O2
 KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
-KBUILD_CFLAGS += $(cflags-y) -fno-delete-null-pointer-checks
+KBUILD_CFLAGS += $(cflags-y) -fno-delete-null-pointer-checks -msoft-float
 KBUILD_CFLAGS += $(call cc-option,-mpacked-stack)
 KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
 

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

queue-4.2/s390-boot-decompression-disable-floating-point-in-decompressor.patch queue-4.2/revert-cgroup-simplify-threadgroup-locking.patch queue-4.2/revert-sched-cgroup-replace-signal_struct-group_rwsem-with-a-global-percpu_rwsem.patch