process: Update pc when trapping from U-mode

This fixes an issue where the same instruction of a U-mode process is executed multiple times if it's scheduled in consecutive time slices.

2 files changed, 33 insertions, 2 deletions

diff --git a/src/interrupts.zig b/src/interrupts.zig
index efde552..ba5f440 100644
--- a/src/interrupts.zig
+++ b/src/interrupts.zig
@@ -90,7 +90,27 @@ pub const SyncCause = enum(u63) {
     _,
 };
 
-export fn handleTrap(epc: usize, tval: usize, cause_bits: usize, hart_id: usize, status: usize, frame: *trap.Frame) usize {
+pub const Sstatus = packed struct(usize) {
+    u_interrupts: u1,
+    s_interrupts: u1,
+    reserved0: u2,
+    u_interrupts_previous: u1,
+    s_interrupts_previous: u1,
+    reserved1: u2,
+    previous_privilege: u1,
+    reserved2: u4,
+    fs: u2,
+    xs: u2,
+    reserved3: u1,
+    sum: u1,
+    mxr: u1,
+    reserved4: u12,
+    u_xlen: u2,
+    reserved5: u29,
+    sd: u1,
+};
+
+export fn handleTrap(epc: usize, tval: usize, cause_bits: usize, hart_id: usize, status: Sstatus, frame: *trap.Frame) usize {
     _ = &status;
     _ = &frame;
 
@@ -102,6 +122,16 @@ export fn handleTrap(epc: usize, tval: usize, cause_bits: usize, hart_id: usize,
         switch (@as(AsyncCause, @enumFromInt(cause.num))) {
             .supervisor_software => w.print("Hart {d}: Software interrupt\r\n", .{hart_id}) catch while (true) {},
             .supervisor_timer => {
+                if (status.previous_privilege == 0) {
+                    // Trapped from U-mode, update pc for next time slice.
+                    //
+                    // We can simply use the last node of the process list here
+                    // because the scheduler moves a process to the end of the queue
+                    // before returning into it.
+
+                    process.list.last.?.data.pc = epc;
+                }
+
                 schedule() catch |err| {
                     std.debug.panic("Hart {d}: Unable to schedule next process: {any}", .{ hart_id, err });
                 };
diff --git a/src/process.zig b/src/process.zig
index 61e42de..5cec65f 100644
--- a/src/process.zig
+++ b/src/process.zig
@@ -12,10 +12,11 @@ const trap = @import("trap.zig");
 
 pub const schedule_interval_millis = 1;
 
+pub var list = std.mem.zeroInit(std.DoublyLinkedList(Info), .{});
+
 const num_stack_pages = 2;
 
 var next_pid: u16 = 1;
-var list = std.mem.zeroInit(std.DoublyLinkedList(Info), .{});
 
 pub const Error = error{
     EmptySchedule,