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authorNicholas Piggin <npiggin@gmail.com>2020-05-11 13:19:52 +0300
committerMichael Ellerman <mpe@ellerman.id.au>2020-07-15 04:08:27 +0300
commit0138ba5783ae0dcc799ad401a1e8ac8333790df9 (patch)
tree3e7ba7e2e19810a618adb8b003aa6c36d8c811fd /arch/powerpc/include/asm/ppc-opcode.h
parentb648a5132ca3237a0f1ce5d871fff342b0efcf8a (diff)
downloadlinux-0138ba5783ae0dcc799ad401a1e8ac8333790df9.tar.xz
powerpc/64/signal: Balance return predictor stack in signal trampoline
Returning from an interrupt or syscall to a signal handler currently begins execution directly at the handler's entry point, with LR set to the address of the sigreturn trampoline. When the signal handler function returns, it runs the trampoline. It looks like this: # interrupt at user address xyz # kernel stuff... signal is raised rfid # void handler(int sig) addis 2,12,.TOC.-.LCF0@ha addi 2,2,.TOC.-.LCF0@l mflr 0 std 0,16(1) stdu 1,-96(1) # handler stuff ld 0,16(1) mtlr 0 blr # __kernel_sigtramp_rt64 addi r1,r1,__SIGNAL_FRAMESIZE li r0,__NR_rt_sigreturn sc # kernel executes rt_sigreturn rfid # back to user address xyz Note the blr with no matching bl. This can corrupt the return predictor. Solve this by instead resuming execution at the signal trampoline which then calls the signal handler. qtrace-tools link_stack checker confirms the entire user/kernel/vdso cycle is balanced after this patch, whereas it's not upstream. Alan confirms the dwarf unwind info still looks good. gdb still recognises the signal frame and can step into parent frames if it break inside a signal handler. Performance is pretty noisy, not a very significant change on a POWER9 here, but branch misses are consistently a lot lower on a microbenchmark: Performance counter stats for './signal': 13,085.72 msec task-clock # 1.000 CPUs utilized 45,024,760,101 cycles # 3.441 GHz 65,102,895,542 instructions # 1.45 insn per cycle 11,271,673,787 branches # 861.372 M/sec 59,468,979 branch-misses # 0.53% of all branches 12,989.09 msec task-clock # 1.000 CPUs utilized 44,692,719,559 cycles # 3.441 GHz 65,109,984,964 instructions # 1.46 insn per cycle 11,282,136,057 branches # 868.585 M/sec 39,786,942 branch-misses # 0.35% of all branches Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20200511101952.1463138-1-npiggin@gmail.com
Diffstat (limited to 'arch/powerpc/include/asm/ppc-opcode.h')
-rw-r--r--arch/powerpc/include/asm/ppc-opcode.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
index 2a39c716c343..bce879fb9afd 100644
--- a/arch/powerpc/include/asm/ppc-opcode.h
+++ b/arch/powerpc/include/asm/ppc-opcode.h
@@ -332,6 +332,7 @@
#define PPC_INST_BLR 0x4e800020
#define PPC_INST_BLRL 0x4e800021
#define PPC_INST_BCTR 0x4e800420
+#define PPC_INST_BCTRL 0x4e800421
#define PPC_INST_MULLD 0x7c0001d2
#define PPC_INST_MULLW 0x7c0001d6
#define PPC_INST_MULHWU 0x7c000016