From 134f7b5fa91fa41e3c3c7caeaf1dff72018684e5 Mon Sep 17 00:00:00 2001
From: Mitchell Horne <mhorne@FreeBSD.org>
Date: Mon, 3 Jun 2024 14:24:12 -0300
Subject: [PATCH] riscv: improve commentary around initial stvec

Make it explicit why we must set the trap vector before enabling virtual
memory.

Reviewed by:	br, jhb, markj
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D45474
---
 sys/riscv/riscv/locore.S | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/sys/riscv/riscv/locore.S b/sys/riscv/riscv/locore.S
index 5a7e15ba443b..7e885ab8ca89 100644
--- a/sys/riscv/riscv/locore.S
+++ b/sys/riscv/riscv/locore.S
@@ -197,7 +197,11 @@ pagetables:
 
 	/* Page tables END */
 
-	/* Setup supervisor trap vector */
+	/*
+	 * Set the supervisor trap vector temporarily. Enabling virtual memory
+	 * may generate a page fault. We simply wish to continue onwards, so
+	 * have the trap deliver us to 'va'.
+	 */
 2:
 	lla	t0, va
 	sub	t0, t0, s9
@@ -221,7 +225,7 @@ va:
 	lla	gp, __global_pointer$
 .option pop
 
-	/* Setup supervisor trap vector */
+	/* Set the trap vector to the real handler. */
 	la	t0, cpu_exception_handler
 	csrw	stvec, t0
 
@@ -342,7 +346,11 @@ ENTRY(mpentry)
 	/* Get the kernel's load address */
 	jal	get_physmem
 
-	/* Setup supervisor trap vector */
+	/*
+	 * Set the supervisor trap vector temporarily. Enabling virtual memory
+	 * may generate a page fault. We simply wish to continue onwards, so
+	 * have the trap deliver us to 'mpva'.
+	 */
 	lla	t0, mpva
 	sub	t0, t0, s9
 	li	t1, KERNBASE
@@ -365,7 +373,7 @@ mpva:
 	lla	gp, __global_pointer$
 .option pop
 
-	/* Setup supervisor trap vector */
+	/* Set the trap vector to the real handler. */
 	la	t0, cpu_exception_handler
 	csrw	stvec, t0