powerpc/pseries/ras: fwnmi sreset should not interlock

PAPR does not specify that fwnmi sreset should be interlocked, and
PowerVM (and therefore now QEMU) do not require it.

These "ibm,nmi-interlock" calls are ignored by firmware, but there
is a possibility that the sreset could have interrupted a machine
check and release the machine check's interlock too early, corrupting
it if another machine check came in.

This is an extremely rare case, but it should be fixed for clarity
and reducing the code executed in the sreset path. Firmware also
does not provide error information for the sreset case to look at, so
remove that comment.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Use __be64 to silence some sparse warnings]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200508043408.886394-9-npiggin@gmail.com

arch/powerpc/platforms/pseries/ras.c

@@ -406,6 +406,20 @@ static inline struct rtas_error_log *fwnmi_get_errlog(void)
 	return (struct rtas_error_log *)local_paca->mce_data_buf;
 }
 
+static __be64 *fwnmi_get_savep(struct pt_regs *regs)
+{
+	unsigned long savep_ra;
+
+	/* Mask top two bits */
+	savep_ra = regs->gpr[3] & ~(0x3UL << 62);
+	if (!VALID_FWNMI_BUFFER(savep_ra)) {
+		printk(KERN_ERR "FWNMI: corrupt r3 0x%016lx\n", regs->gpr[3]);
+		return NULL;
+	}
+
+	return __va(savep_ra);
+}
+
 /*
  * Get the error information for errors coming through the
  * FWNMI vectors.  The pt_regs' r3 will be updated to reflect
@@ -423,20 +437,14 @@ static inline struct rtas_error_log *fwnmi_get_errlog(void)
  */
 static struct rtas_error_log *fwnmi_get_errinfo(struct pt_regs *regs)
 {
-	unsigned long savep_ra;
-	unsigned long *savep;
 	struct rtas_error_log *h;
+	__be64 *savep;
 
-	/* Mask top two bits */
-	savep_ra = regs->gpr[3] & ~(0x3UL << 62);
-
-	if (!VALID_FWNMI_BUFFER(savep_ra)) {
-		printk(KERN_ERR "FWNMI: corrupt r3 0x%016lx\n", regs->gpr[3]);
+	savep = fwnmi_get_savep(regs);
+	if (!savep)
 		return NULL;
-	}
 
-	savep = __va(savep_ra);
-	regs->gpr[3] = be64_to_cpu(savep[0]);	/* restore original r3 */
+	regs->gpr[3] = be64_to_cpu(savep[0]); /* restore original r3 */
 
 	h = (struct rtas_error_log *)&savep[1];
 	/* Use the per cpu buffer from paca to store rtas error log */
@@ -483,11 +491,21 @@ int pSeries_system_reset_exception(struct pt_regs *regs)
 #endif
 
 	if (fwnmi_active) {
-		struct rtas_error_log *errhdr = fwnmi_get_errinfo(regs);
-		if (errhdr) {
-			/* XXX Should look at FWNMI information */
-		}
-		fwnmi_release_errinfo();
+		__be64 *savep;
+
+		/*
+		 * Firmware (PowerVM and KVM) saves r3 to a save area like
+		 * machine check, which is not exactly what PAPR (2.9)
+		 * suggests but there is no way to detect otherwise, so this
+		 * is the interface now.
+		 *
+		 * System resets do not save any error log or require an
+		 * "ibm,nmi-interlock" rtas call to release.
+		 */
+
+		savep = fwnmi_get_savep(regs);
+		if (savep)
+			regs->gpr[3] = be64_to_cpu(savep[0]); /* restore original r3 */
 	}
 
 	if (smp_handle_nmi_ipi(regs))