From 4fa20f17e0363957ed519d9abfe28a43256220b5 Mon Sep 17 00:00:00 2001
From: Nathan Ringo <nathan@remexre.com>
Date: Sat, 3 Aug 2024 14:01:53 -0500
Subject: uncommitted changes...

---
 boards/qemu-virt/qemu-virt.s | 64 ++++++++++++++++++++++++++++++--------------
 kernel/src/lib.rs            |  2 +-
 2 files changed, 45 insertions(+), 21 deletions(-)

diff --git a/boards/qemu-virt/qemu-virt.s b/boards/qemu-virt/qemu-virt.s
index a2618d2..5c58153 100644
--- a/boards/qemu-virt/qemu-virt.s
+++ b/boards/qemu-virt/qemu-virt.s
@@ -1,14 +1,15 @@
-.section .text.start
-
 .extern main
 .extern CONSOLE_STRICT_FLUSH
 
+.section .text.start
+
 .global _start
 .type _start, STT_FUNC
 _start:
-	# Have harts other than 0 spin until hart0 wakes them up.
-	csrr a0, mhartid
-	c.bnez a0, wait_for_hart0
+	# Have harts other than 0 spin until hart0 wakes them up. As a side
+	# effect, load every hart's thread pointer register with MHARTID.
+	csrr tp, mhartid
+	bnez tp, wait_for_hart0
 
 	# Set up console_strict_flush.
 	la t0, console_strict_flush
@@ -18,16 +19,47 @@ _start:
 	# Set up hart0's stack.
 	la sp, hart0_initial_stack_top
 
-	# Call hart0_boot with the address of the DeviceTree.
+	# Put the address of the DeviceTree into the first argument position
+	# for hart0_boot.
 	c.mv a0, a1
-	call hart0_boot
-	# Fall through to a spin loop.
+
+	# The rest of this function is to get out of machine mode and into
+	# supervisor mode by fictitiously returning from a trap.
+	
+	# First, set MSTATUS.MPP to Supervisor, so that when we execute mret,
+	# the privilege level will lower to Supervisor.
+	csrr t0, mstatus
+	li t1, ~(0b11 << 11)
+	and t0, t0, t1
+	li t1,  (0b01 << 11)
+	or  t0, t0, t1
+	csrw mstatus, t0
+
+	# Next, set MEPC to the address of hart0_boot, so that when we execute
+	# mret, execution will continue there.
+	la t0, hart0_boot
+	csrw mepc, t0
+
+	# Delegate all exceptions and interrupts to the Supervisor level.
+	li t0, 0xffff
+	csrw medeleg, t0
+	csrw mideleg, t0
+	
+	# Set SATP.MODE to Bare, and clear the rest of the bits. This disables
+	# paging in Supervisor mode.
+	csrw satp, zero
+
+	mret
 .size _start, . - _start
 
-.type halt, STT_FUNC
-halt:
-	j halt
-.size halt, . - halt
+.section .text
+
+.type wait_for_hart0, STT_FUNC
+wait_for_hart0:
+	# TODO
+	wfi
+	j wait_for_hart0
+.size wait_for_hart0, . - wait_for_hart0
 
 .section .text
 
@@ -44,11 +76,3 @@ console_strict_flush:
 2:
 	ret
 .size console_strict_flush, . - console_strict_flush
-
-.section .text
-
-.type wait_for_hart0, STT_FUNC
-wait_for_hart0:
-	# TODO
-	j halt
-.size wait_for_hart0, . - wait_for_hart0
diff --git a/kernel/src/lib.rs b/kernel/src/lib.rs
index 4a2483a..842609f 100644
--- a/kernel/src/lib.rs
+++ b/kernel/src/lib.rs
@@ -9,7 +9,7 @@ mod panic;
 /// The entrypoint to the kernel. This should be executed by hart0 alone. It performs some early
 /// boot tasks, then wakes up any other harts.
 #[no_mangle]
-pub extern "C" fn hart0_boot(device_tree: *const u32) {
+pub extern "C" fn hart0_boot(device_tree: *const u32) -> ! {
     console::init();
 
     log::info!("device_tree = {device_tree:?}");
-- 
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