runtime: Save G on signal stack for PPC VDSO call

Author: derekparker

src/runtime/sys_linux_ppc64x.s

@@ -216,29 +216,58 @@ TEXT runtime·walltime(SB),NOSPLIT,$16-12
 	MOVD	(g_sched+gobuf_sp)(R7), R1	// Set SP to g0 stack
 
 noswitch:
-	SUB	$16, R1			// Space for results
-	RLDICR	$0, R1, $59, R1		// Align for C code
-	MOVD	R12, CTR
-	MOVD	R1, R4
-	BL	(CTR)			// Call from VDSO
-	MOVD	$0, R0			// Restore R0
-	MOVD	0(R1), R3		// sec
-	MOVD	8(R1), R5		// nsec
-	MOVD	R15, R1			// Restore SP
+	SUB     $16, R1                 // Space for results
+	RLDICR  $0, R1, $59, R1         // Align for C code
+	MOVD    R12, CTR
+	MOVD    R1, R4
+
+	// Store g on gsignal's stack, so if we receive a signal
+	// during VDSO code we can find the g.
+	// If we don't have a signal stack, we won't receive signal,
+	// so don't bother saving g.
+	// When using cgo, we already saved g on TLS, also don't save
+	// g here.
+	// Also don't save g if we are already on the signal stack.
+	// We won't get a nested signal.
+	MOVBZ   runtime·iscgo(SB), R22
+	CMP     R22, $0
+	BNE    nosaveg
+	MOVD    m_gsignal(R21), R22          // g.m.gsignal
+	CMP     R22, $0
+	BEQ     nosaveg
+
+	CMP     g, R22
+	BEQ     nosaveg
+	MOVD    (g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
+	MOVD    g, (R22)
+
+	BL      (CTR)                   // Call from VDSO
+
+	MOVD    $0, (R22)  // clear g slot, R22 is unchanged by C code
+
+	BL       finish
+
+nosaveg:
+	BL      (CTR)                   // Call from VDSO
+
+finish:
+	MOVD    $0, R0                  // Restore R0
+	MOVD    0(R1), R3               // sec
+	MOVD    8(R1), R5               // nsec
+	MOVD    R15, R1                 // Restore SP
 
 	// Restore vdsoPC, vdsoSP
 	// We don't worry about being signaled between the two stores.
 	// If we are not in a signal handler, we'll restore vdsoSP to 0,
 	// and no one will care about vdsoPC. If we are in a signal handler,
 	// we cannot receive another signal.
-	MOVD	40(R1), R6
-	MOVD	R6, m_vdsoSP(R21)
-	MOVD	32(R1), R6
-	MOVD	R6, m_vdsoPC(R21)
+	MOVD    40(R1), R6
+	MOVD    R6, m_vdsoSP(R21)
+	MOVD    32(R1), R6
+	MOVD    R6, m_vdsoPC(R21)
 
-finish:
-	MOVD	R3, sec+0(FP)
-	MOVW	R5, nsec+8(FP)
+	MOVD    R3, sec+0(FP)
+	MOVW    R5, nsec+8(FP)
 	RET
 
 	// Syscall fallback
@@ -469,7 +498,7 @@ TEXT sigtramp<>(SB),NOSPLIT|NOFRAME,$0
 	// this might be called in external code context,
 	// where g is not set.
 	MOVBZ	runtime·iscgo(SB), R6
-	CMP 	R6, $0
+	CMP	R6, $0
 	BEQ	2(PC)
 	BL	runtime·load_g(SB)