Patch "ARM: 8634/1: hw_breakpoint: blacklist Scorpion CPUs" has been added to the 4.9-stable tree

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

ARM: 8634/1: hw_breakpoint: blacklist Scorpion CPUs

to the 4.9-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:
arm-8634-1-hw_breakpoint-blacklist-scorpion-cpus.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Mark Rutland <[email protected]>
Date: Fri, 6 Jan 2017 13:12:47 +0100
Subject: ARM: 8634/1: hw_breakpoint: blacklist Scorpion CPUs

From: Mark Rutland <[email protected]>

commit ddc37832a1349f474c4532de381498020ed71d31 upstream.

On APQ8060, the kernel crashes in arch_hw_breakpoint_init, taking an undefined instruction trap within write_wb_reg. This is because Scorpion CPUs erroneously appear to set DBGPRSR.SPD when WFI is issued, even if the core is not powered down. When DBGPRSR.SPD is set, breakpoint and watchpoint registers are treated as undefined.

It's possible to trigger similar crashes later on from userspace, by requesting the kernel to install a breakpoint or watchpoint, as we can go idle at any point between the reset of the debug registers and their later use. This has always been the case.

Given that this has always been broken, no-one has complained until now, and there is no clear workaround, disable hardware breakpoints and watchpoints on Scorpion to avoid these issues.

---
 arch/arm/include/asm/cputype.h  |    3 +++
 arch/arm/kernel/hw_breakpoint.c |   16 ++++++++++++++++
 2 files changed, 19 insertions(+)

@@ -94,6 +94,9 @@
 #define ARM_CPU_XSCALE_ARCH_V2		0x4000
 #define ARM_CPU_XSCALE_ARCH_V3		0x6000
 
+/* Qualcomm implemented cores */
+#define ARM_CPU_PART_SCORPION		0x510002d0
+
 extern unsigned int processor_id;
 
 #ifdef CONFIG_CPU_CP15

@@ -1066,6 +1066,22 @@ static int __init arch_hw_breakpoint_ini
 		return 0;
 	}
 
+	/*
+	 * Scorpion CPUs (at least those in APQ8060) seem to set DBGPRSR.SPD
+	 * whenever a WFI is issued, even if the core is not powered down, in
+	 * violation of the architecture.  When DBGPRSR.SPD is set, accesses to
+	 * breakpoint and watchpoint registers are treated as undefined, so
+	 * this results in boot time and runtime failures when these are
+	 * accessed and we unexpectedly take a trap.
+	 *
+	 * It's not clear if/how this can be worked around, so we blacklist
+	 * Scorpion CPUs to avoid these issues.
+	*/
+	if (read_cpuid_part() == ARM_CPU_PART_SCORPION) {
+		pr_info("Scorpion CPU detected. Hardware breakpoints and watchpoints disabled\n");
+		return 0;
+	}
+
 	has_ossr = core_has_os_save_restore();
 
 	/* Determine how many BRPs/WRPs are available. */

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

queue-4.9/arm-8634-1-hw_breakpoint-blacklist-scorpion-cpus.patch queue-4.9/arm64-avoid-returning-from-bad_mode.patch