From e1bd496b8faf26081cb008dd308f906ca46d8275 Mon Sep 17 00:00:00 2001
From: Mark Poliakov <mark@alnyan.me>
Date: Wed, 26 Mar 2025 14:36:04 +0200
Subject: [PATCH] WIP: Enter kernel tasks via exception return

---
 src/arch/aarch64/context.S | 20 +++++++++++++++++---
 src/arch/riscv64/context.S | 11 +++++++++--
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/arch/aarch64/context.S b/src/arch/aarch64/context.S
index b70d10e..10aafb8 100644
--- a/src/arch/aarch64/context.S
+++ b/src/arch/aarch64/context.S
@@ -29,6 +29,9 @@
 
 .pushsection .text
 
+.set SPSR_ELx_I,    (1 << 9)
+.set SPSR_ELx_EL1h, (0b0101)
+
 __aa64_task_enter_user:
     // x0 == sp, ...
     ldr x0, [sp, #16 * 0]
@@ -40,7 +43,8 @@ __aa64_task_enter_user:
 
     msr elr_el1, x1
 
-    mov x1, #(1 << 9)
+    // SPSR_ELx_M[4:0] = 0, a return to EL0, AArch64 mode
+    mov x1, #SPSR_ELx_I
     msr spsr_el1, x1
 
     mov lr, xzr
@@ -57,9 +61,19 @@ __aa64_task_enter_kernel:
     ldr lr, [sp, #16]
     add sp, sp, #24
 
-    // TODO enter task via eret to EL1t
+    msr elr_el1, x1
 
-    br x1
+    // SPSR_ELx_M[4:0] = 0b100, a return to EL1t, AArch64 mode
+    mov x1, #SPSR_ELx_EL1h
+    orr x1, x1, #SPSR_ELx_I
+    msr spsr_el1, x1
+
+    mov x1, xzr
+
+    dsb ish
+    isb sy
+
+    eret
 
 __aa64_switch_task:
     // x0 -- "dst" context
diff --git a/src/arch/riscv64/context.S b/src/arch/riscv64/context.S
index e743774..ae04e7b 100644
--- a/src/arch/riscv64/context.S
+++ b/src/arch/riscv64/context.S
@@ -74,8 +74,15 @@ __rv64_task_enter_kernel:
     ld ra, 16(sp)   // return address
     addi sp, sp, 24
 
-    // TODO S-mode -> S-mode return via sret
-    jr t0
+    // Set SPP to indicate a return to S-mode
+    csrr t1, sstatus
+    // TODO enable interrupts via SPIE
+    // ori t0, t0, SSTATUS_SPIE
+    ori t1, t1, SSTATUS_SPP
+    csrw sstatus, t1
+    csrw sepc, t0
+
+    sret
 
 __rv64_switch_task:
     // a0 - new context