instructions: Represent sstatus CSR in the type system

2 files changed, 43 insertions, 10 deletions

diff --git a/src/lib/instructions.zig b/src/lib/instructions.zig
index 81775b2..d072113 100644
--- a/src/lib/instructions.zig
+++ b/src/lib/instructions.zig
@@ -7,6 +7,44 @@ const std = @import("std");
 const interrupts = @import("interrupts.zig");
 const paging = @import("paging.zig");
 
+pub const Privilege = enum(u1) {
+    user = 0,
+    supervisor = 1,
+};
+
+pub const ExtensionState = enum(u2) {
+    off = 0,
+    initial = 1,
+    clean = 2,
+    dirty = 3,
+};
+
+pub const Xlen = enum(u2) {
+    rv32 = 1,
+    rv64 = 2,
+    rv128 = 3,
+};
+
+pub const Sstatus = packed struct(usize) {
+    user_interrupt_enable: u1,
+    supervisor_interrupt_enable: u1,
+    reserved0: u2,
+    user_prior_interrupt_enable: u1,
+    supervisor_prior_interrupt_enable: u1,
+    reserved1: u2,
+    previous_privilege: Privilege,
+    reserved2: u4,
+    floating_point_state: ExtensionState,
+    user_extension_state: ExtensionState,
+    reserved3: u1,
+    supervisor_user_memory_access: u1,
+    make_executable_readable: u1,
+    reserved4: u12,
+    user_xlen: Xlen,
+    reserved5: u29,
+    need_state_saving: u1, // Read-only.
+};
+
 pub const SbiRet = struct {
     err: isize,
     val: isize,
@@ -46,6 +84,7 @@ pub fn stackPointer() usize {
 }
 
 pub const satp = Csr(paging.Satp, "satp");
+pub const sstatus = Csr(Sstatus, "sstatus");
 pub const sie = Csr(interrupts.Enable, "sie");
 pub const sscratch = Csr(usize, "sscratch");
 pub const sepc = Csr(usize, "sepc");
diff --git a/src/lib/process.zig b/src/lib/process.zig
index 05c6773..5392229 100644
--- a/src/lib/process.zig
+++ b/src/lib/process.zig
@@ -95,13 +95,9 @@ pub fn switchTo(proc: *Info) noreturn {
 
     instructions.sscratch.write(@intFromPtr(&proc.trap_frame));
 
-    asm volatile (
-        \\ csrr t0, sstatus
-        \\ li t1, 0x100
-        \\ not t1, t1
-        \\ and t0, t0, t1
-        \\ csrw sstatus, t0
-        ::: "t0", "t1");
+    var sstatus = instructions.sstatus.read();
+    sstatus.previous_privilege = .user;
+    instructions.sstatus.write(sstatus);
 
     instructions.sepc.write(proc.pc);
     instructions.satp.write(proc.satp());
@@ -152,7 +148,7 @@ pub fn switchTo(proc: *Info) noreturn {
     unreachable;
 }
 
-pub fn demo(allocator: std.mem.Allocator) !void {
+pub fn demo(allocator: std.mem.Allocator) !noreturn {
     const entry: [*]u8 = @alignCast(@ptrCast(try paging.zeroedAlloc(1)));
     defer paging.free(@ptrCast(entry)) catch {};
 
@@ -168,6 +164,4 @@ pub fn demo(allocator: std.mem.Allocator) !void {
 
     try time.interruptInMillis(schedule_interval_millis);
     try switchTo(&proc_node.data);
-
-    while (true) asm volatile ("wfi");
 }