Commit 70fe3d9 "powerpc: Restore FPU/VEC/VSX if previously used" introduces a
call to restore_math() late in the syscall return path, after MSR_RI has been
cleared. The MSR_RI flag is used to indicate whether the kernel can take
another exception or not. A cleared MSR_RI flag indicates that the kernel
cannot.
Unfortunately when a machine is under high load an SLB miss can occur in
restore_math() which (with MSR_RI cleared) leads to an unrecoverable exception.
Unrecoverable exception trace:
powerpc: Restore FPU/VEC/VSX if previously used
Unrecoverable exception 4100 at c0000000000088d8
cpu 0x0: Vector: 4100 at [c0000003fa473b20]
pc: c0000000000088d8: .load_vr_state+0x70/0x110
lr: c00000000000f710: .restore_math+0x130/0x188
sp: c0000003fa473da0
msr: 9000000002003030
current = 0xc0000007f876f180
paca = 0xc00000000fff0000 softe: 0 irq_happened: 0x01
pid = 1944, comm = K08umountfs
Linux version 4.5.0-rc3-g22ccd98 (kerkins@alpine1-p1) (gcc version 5.2.1
20151001 (GCC) ) #1 SMP Tue Mar 15 21:33:26 AEDT 2016
WARNING: exception is not recoverable, can't continue
enter ? for help
[link register ] c00000000000f710 .restore_math+0x130/0x188
[c0000003fa473da0] c0000003fa473e30 (unreliable)
[c0000003fa473e30] c000000000007b6c system_call+0x84/0xfc
--- Exception: c00 (System Call) at 000000000fe84328
0:mon>
The clearing of MSR_RI is actually an optimisation to avoid multiple MSR
writes, what must be disabled are interrupts. See comment in entry_64.S:
/*
* For performance reasons we clear RI the same time that we
* clear EE. We only need to clear RI just before we restore r13
* below, but batching it with EE saves us one expensive mtmsrd call.
* We have to be careful to restore RI if we branch anywhere from
* here (eg syscall_exit_work).
*/
At the point of calling restore_math() r13 has not been restored, as such, the
quick fix of turning MSR_RI back on for the call to restore_math() will
eliminate the occurrence of an unrecoverable exception.
We'd like to do a better fix in future.
Signed-off-by: Cyril Bur <cyril...@gmail.com>
---
arch/powerpc/kernel/entry_64.S | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 038e0a1..f3aa4b4 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -218,7 +218,16 @@ system_call: /* label this so stack
traces look sane */
bne 3f
#endif
2: addi r3,r1,STACK_FRAME_OVERHEAD
+#ifdef CONFIG_PPC_BOOK3S
+ mtmsrd r10,1 /* Restore RI */
+#endif
bl restore_math
+#ifdef CONFIG_PPC_BOOK3S
+ ld r10,PACAKMSR(r13)
+ li r9,MSR_RI
+ andc r11,r10,r9 /* Re-clear RI */
+ mtmsrd r11,1
+#endif
ld r8,_MSR(r1)
ld r3,RESULT(r1)
li r11,-MAX_ERRNO
--
2.7.3
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
