Hi Mark,

On Thu, Mar 31, 2016 at 11:05:48AM +0100, Mark Rutland wrote:

On Thu, Mar 31, 2016 at 05:27:03AM +0300, Yury Norov wrote:

quoted

Not all vendors implement all the system registers ARM specifies.

The ID registers in question are precisely documented in the ARM ARM
(see table C5-6 in ARM DDI 0487A.i). Specifically, the ID space
ID_AA64MMFR2_EL1 now falls in to is listed as RAZ.

Any deviation from this is an erratum, and needs to be handled as such
(e.g. listing in silicon-errata.txt).

Does the issue affect ThunderX natively?

Yes, Thunder is involved, but I cannot tell more due to NDA.
And this error is not in silicon-errata.txt.
I'll ask permission to share more details.

Or has this only been seen with QEMU (in TCG mode), where the bug has
already been fixed?

quoted

So access them causes undefined instruction abort and then kernel
panic. There are 3 ways to handle it we can figure out:
 - use conditional compilation and erratas;
 - use kernel patching;
 - inline fixups resolving the abort.

Last option is more robust as it does not require additional efforts
to support targers. It is looking reasonable because in many cases
optional registers should be RAZ if not implemented. Special cases may
be handled by underlying __read_cpuid() when needed.

I don't think we should do this if the only affected implementations are
software emulators which can be patched (and have already been, in the
case of QEMU).

In future it's very likely that early assembly code (potentially in
hypervisor context) will need to access ID registers which are currently
reserved/RAZ, and it will be rather painful to fix up accesses to this.

So we will not fix. This one fixes el1 only, and don't pretend for more. 

Additionally, this workaround will silently mask other bugs in this area
(e.g. if registers like ID_AA64MMFR0_EL1 were to trap for some reason on
an implementation), which doesn't seem good.

We can mask it less silently, for example, print message to dmesg.

Initially I was thinking about erratas as well, but Arnd suggested
this approach, and now think it's better. From consumer point of view,
it's much better to have a warning line in dmesg, instead of bricked
device, after another kernel or driver update.

Thanks,
Mark.

quoted

Tested on QEMU emulator version 2.3.0 (Debian 1:2.3+dfsg-5ubuntu9.2)
that does not implement SYS_ID_AA64MMFR2_EL1 register. (Fixed in new
releases),

Discussion: https://lkml.org/lkml/2016/3/29/931

Signed-off-by: Yury Norov <redacted>
---
 arch/arm64/include/asm/cputype.h  | 17 +++++++++++++++--
 arch/arm64/include/asm/exttable.h | 21 +++++++++++++++++++++
 arch/arm64/include/asm/uaccess.h  | 15 ---------------
 arch/arm64/kernel/entry.S         |  3 ++-
 arch/arm64/kernel/traps.c         |  6 ++++++
 arch/arm64/mm/extable.c           |  2 +-
 6 files changed, 45 insertions(+), 19 deletions(-)
 create mode 100644 arch/arm64/include/asm/exttable.h

diff --git a/arch/arm64/include/asm/cputype.h b/arch/arm64/include/asm/cputype.h
index 87e1985..9660130 100644
--- a/arch/arm64/include/asm/cputype.h
+++ b/arch/arm64/include/asm/cputype.h

@@ -90,13 +90,26 @@
 #ifndef __ASSEMBLY__
 
 #include <asm/sysreg.h>
+#include <asm/exttable.h>
 
-#define read_cpuid(reg) ({						\
+#define __read_cpuid(reg, err) ({					\
 	u64 __val;							\
-	asm("mrs_s	%0, " __stringify(SYS_ ## reg) : "=r" (__val));	\
+	asm (								\
+	"1:mrs_s	%0, " reg "\n"					\
+	"2:\n"								\
+	"	.section .fixup, \"ax\"\n"				\
+	"	.align	2\n"						\
+	"3:	mov	%w0, %1\n"					\
+	"	b	2b\n"						\
+	"	.previous\n"						\
+	_ASM_EXTABLE(1b, 3b)						\
+	: "=r" (__val)							\
+	: "i" (err));							\
 	__val;								\
 })
 
+#define read_cpuid(reg) __read_cpuid(__stringify(SYS_ ## reg), 0)
+
 /*
  * The CPU ID never changes at run time, so we might as well tell the
  * compiler that it's constant.  Use this function to read the CPU ID

diff --git a/arch/arm64/include/asm/exttable.h b/arch/arm64/include/asm/exttable.h
new file mode 100644
index 0000000..c0a66c8
--- /dev/null
+++ b/arch/arm64/include/asm/exttable.h

@@ -0,0 +1,21 @@
+#ifndef __ASM_EXTTABLE_H
+#define __ASM_EXTTABLE_H
+
+#include <asm/ptrace.h>
+
+#define ARCH_HAS_RELATIVE_EXTABLE
+
+#define _ASM_EXTABLE(from, to)						\
+	"	.pushsection	__ex_table, \"a\"\n"			\
+	"	.align		3\n"					\
+	"	.long		(" #from " - .), (" #to " - .)\n"	\
+	"	.popsection\n"
+
+struct exception_table_entry
+{
+	int insn, fixup;
+};
+
+extern int fixup_exception(struct pt_regs *regs);
+
+#endif /* __ASM_EXTTABLE_H */

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 0685d74..19cfdc5 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h

@@ -48,15 +48,6 @@
  * on our cache or tlb entries.
  */
 
-struct exception_table_entry
-{
-	int insn, fixup;
-};
-
-#define ARCH_HAS_RELATIVE_EXTABLE
-
-extern int fixup_exception(struct pt_regs *regs);
-
 #define KERNEL_DS	(-1UL)
 #define get_ds()	(KERNEL_DS)
 

@@ -117,12 +108,6 @@ static inline void set_fs(mm_segment_t fs)
 #define access_ok(type, addr, size)	__range_ok(addr, size)
 #define user_addr_max			get_fs
 
-#define _ASM_EXTABLE(from, to)						\
-	"	.pushsection	__ex_table, \"a\"\n"			\
-	"	.align		3\n"					\
-	"	.long		(" #from " - .), (" #to " - .)\n"	\
-	"	.popsection\n"
-
 /*
  * The "__xxx" versions of the user access functions do not verify the address
  * space - it must have been done previously with a separate "access_ok()"

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 1f7a145..6b88096 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -377,7 +377,8 @@ el1_undef:
 	 */
 	enable_dbg
 	mov	x0, sp
-	b	do_undefinstr
+	bl	do_undefinstr
+	kernel_exit 1
 el1_dbg:
 	/*
 	 * Debug exception handling

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index cacd30a..515444a 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c

@@ -386,6 +386,12 @@ asmlinkage void __exception do_undefinstr(struct pt_regs *regs)
 	if (call_undef_hook(regs) == 0)
 		return;
 
+	/*
+	 * Are we prepared to handle this kernel fault?
+	 */
+	if (fixup_exception(regs))
+		return;
+
 	if (unhandled_signal(current, SIGILL) && show_unhandled_signals_ratelimited()) {
 		pr_info("%s[%d]: undefined instruction: pc=%p\n",
 			current->comm, task_pid_nr(current), pc);

diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
index 81acd47..c140688 100644
--- a/arch/arm64/mm/extable.c
+++ b/arch/arm64/mm/extable.c

@@ -3,7 +3,7 @@
  */
 
 #include <linux/module.h>
-#include <linux/uaccess.h>
+#include <asm/exttable.h>
 
 int fixup_exception(struct pt_regs *regs)
 {

-- 
2.5.0


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