From 954a379c2dcbbee4e623c73dc14b48e2d02fd128 Mon Sep 17 00:00:00 2001
From: Amoelle <amoelle@proton.me>
Date: Wed, 23 Jul 2025 15:52:18 +0300
Subject: [PATCH] finish implementation of running multiple processes
 concurrently (unsafely)

---
 kernel.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel.c b/kernel.c
index e55a6f5..a5a1255 100644
--- a/kernel.c
+++ b/kernel.c
@@ -43,7 +43,9 @@ __attribute__((naked))
 __attribute__((aligned(4)))
 void kernel_entry(void) {
     __asm__ __volatile__(
-        "csrw sscratch, sp\n"
+        // Retrieve the kernel stack of the running process from sscratch
+        "csrrw sp, sscratch, sp\n"
+
         "addi sp, sp, -4 * 31\n"
         "sw ra, 4 * 0(sp)\n"
         "sw gp, 4 * 1(sp)\n"
@@ -76,9 +78,11 @@ void kernel_entry(void) {
         "sw s10, 4 * 28(sp)\n"
         "sw s11, 4 * 29(sp)\n"
 
+        // Retrieve and save the sp at the time of exception
         "csrr a0, sscratch\n"
         "sw a0, 4 * 30(sp)\n"
 
+        // Reset the kernel stack
         "mv a0, sp\n"
         "call handle_trap\n"
 
@@ -230,6 +234,12 @@ void yield(void) {
     if (next == current_proc)
         return;
 
+    __asm__ __volatile__(
+        "csrw sscratch, %[sscratch]\n"
+        :
+        : [sscratch] "r" ((uint32_t) &next->stack[sizeof(next->stack)])
+    );
+
     // Context switch
     struct process *prev = current_proc;
     current_proc = next;