[Skiboot] [PATCH 3/8] cpu: add debug check in cpu_relax

[Nicholas Piggin]

If cpu_relax() is called when not at medium SMT priority, it will lose
the prior priority and return at medium. Add a debug check to catch
this, which would have flagged the previous bug.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 core/cpu.c          | 6 ++++++
 include/processor.h | 1 +
 2 files changed, 7 insertions(+)

diff --git a/core/cpu.c b/core/cpu.c
index 5c10fc6e8..0f2da1524 100644
--- a/core/cpu.c
+++ b/core/cpu.c
@@ -80,6 +80,12 @@ unsigned long __attrconst cpu_emergency_stack_top(unsigned int pir)
 
 void __nomcount cpu_relax(void)
 {
+	if ((mfspr(SPR_PPR32) >> 18) != 0x4) {
+		printf("cpu_relax called when not at medium SMT priority: "
+			"PPR[PRI]=0x%lx\n", mfspr(SPR_PPR32) >> 18);
+		backtrace();
+	}
+
 	/* Relax a bit to give sibling threads some breathing space */
 	smt_lowest();
 	asm volatile("nop; nop; nop; nop;\n"
diff --git a/include/processor.h b/include/processor.h
index 973d7e77b..7a9c49994 100644
--- a/include/processor.h
+++ b/include/processor.h
@@ -71,6 +71,7 @@
 #define SPR_USRR1	0x1fb   /* RW: Ultravisor Save/Restore Register 1 */
 #define SPR_SMFCTRL	0x1ff   /* RW: Secure Memory Facility Control */
 #define SPR_PSSCR	0x357   /* RW: Stop status and control (ISA 3) */
+#define SPR_PPR32	0x382
 #define SPR_TSCR	0x399
 #define SPR_HID0	0x3f0
 #define SPR_HID1	0x3f1
-- 
2.23.0