linux-kernel - [PATCH 3.16.y-ckt 04/94] s390/boot/decompression: disable floating point in decompressor

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Message-Id: <1447409428-12178-5-git-send-email-luis.henriques@canonical.com>
Date:	Fri, 13 Nov 2015 10:08:58 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 04/94] s390/boot/decompression: disable floating point in decompressor

3.16.7-ckt20 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christian Borntraeger <borntraeger@...ibm.com>

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.

Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@...ibm.com>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 arch/s390/boot/compressed/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/boot/compressed/Makefile b/arch/s390/boot/compressed/Makefile
index f90d1fc6d603..f70b2321071e 100644
--- a/arch/s390/boot/compressed/Makefile
+++ b/arch/s390/boot/compressed/Makefile
@@ -12,7 +12,7 @@ targets += misc.o piggy.o sizes.h head$(BITS).o
 
 KBUILD_CFLAGS := -m$(BITS) -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)
 
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