feat(ke): logging and consoles

src/kern/log.zig

@@ -1,33 +1,52 @@
 const std = @import("std");
 const b = @import("base");
+const rtl = @import("rtl");
 const pl = b.pl;
 const ke = b.ke;
+const ki = ke.private;
 
-pub var debug_lock = ke.SpinLock.init();
+var console_lock = ke.SpinLock.init();
+var console_list: rtl.List = .{ .head = .{ .next = &console_list.head, .prev = &console_list.head } };
 
-pub fn print(bytes: []const u8) void {
+/// Represents a console device.
-    for (bytes) |c| {
+/// This is used as an output device.
-        pl.debug_write(c);
+pub const Console = struct {
-    }
+    last_seq: u64,
+    write: WriteFn,
+    link: rtl.List.Entry,
+    pub const WriteFn = *const fn (self: *Console, data: []const u8) void;
+};
+
+/// Register a console.
+pub fn register_console(console: *Console) void {
+    const ipl = console_lock.acquire();
+    console_list.insert_tail(&console.link);
+    console_lock.release(ipl);
+
+    flush();
 }
 
-const LogWriter = struct {
+const ConsoleWriter = struct {
     interface: std.Io.Writer,
+    console: *Console,
 
-    pub fn init() LogWriter {
+    pub fn init(console: *Console) ConsoleWriter {
         return .{
             .interface = .{
                 .vtable = &.{ .drain = drain },
                 .buffer = &.{},
                 .end = 0,
             },
+            .console = console,
         };
     }
 
-    fn drain(_: *std.Io.Writer, data: []const []const u8, _: usize) std.Io.Writer.Error!usize {
+    fn drain(w: *std.Io.Writer, data: []const []const u8, _: usize) std.Io.Writer.Error!usize {
+        const self: *ConsoleWriter = @fieldParentPtr("interface", w);
+
         var total_written: usize = 0;
         for (data) |slice| {
-            print(slice);
+            self.console.write(self.console, slice);
             total_written += slice.len;
         }
 
@@ -35,6 +54,34 @@ const LogWriter = struct {
     }
 };
 
+var ringbuffer = ki.log_ring.RingBuffer(14, 5).init();
+
+fn flush() void {
+    var buf: [512]u8 = undefined;
+
+    // Note: we trylock here because if the lock is already held, that means
+    // someone else is already flushing the logs.
+    if (console_lock.try_acquire()) |ipl| {
+        var it = console_list.iterator();
+
+        while (it.next()) : (it.advance()) {
+            const console: *Console = @fieldParentPtr("link", it.get());
+            var writer = ConsoleWriter.init(console);
+
+            while (true) {
+                const info = ringbuffer.read(console.last_seq, &buf) catch break;
+
+                writer.interface.print("[{:>5}.{:06}] ", .{ info.timestamp / std.time.ns_per_s, info.timestamp / std.time.ns_per_us }) catch break;
+                console.write(console, buf[0..info.length]);
+
+                console.last_seq = info.sequence + 1;
+            }
+        }
+
+        console_lock.release(ipl);
+    }
+}
+
 pub fn log(
     comptime level: std.log.Level,
     comptime scope: @EnumLiteral(),
@@ -44,14 +91,17 @@ pub fn log(
     _ = level;
     _ = scope;
 
-    const ipl = debug_lock.acquire();
+    var discarder: std.Io.Writer.Discarding = .init(&.{});
+    discarder.writer.print(fmt ++ "\n", args) catch return;
 
-    var writer = LogWriter.init();
+    var res = ringbuffer.reserve(discarder.fullCount()) catch return;
 
-    writer.interface.print(
+    res.info.timestamp = ke.timecounter.read_time_nano();
-        fmt ++ "\r\n",
+    res.info.length = @truncate(discarder.fullCount());
-        args,
-    ) catch unreachable;
 
-    debug_lock.release(ipl);
+    res.buf = std.fmt.bufPrint(res.buf, fmt ++ "\n", args) catch return;
+
+    ringbuffer.publish(res);
+
+    flush();
 }

src/kern/log_ring.zig

@@ -0,0 +1,566 @@
+const std = @import("std");
+const base = @import("base");
+const rtl = @import("rtl");
+const ke = base.ke;
+const ki = ke.private;
+
+const BlkPos = struct {
+    begin: usize,
+    end: usize,
+};
+
+const DescState = enum(u2) {
+    Reserved = 0,
+    Published = 1,
+    Free = 2,
+    Miss = 3,
+};
+
+const usize_bits = @bitSizeOf(usize);
+// Id is a monotonically increasing counter. `id & desc_mask` maps to the physical array index.
+const Id = std.meta.Int(.unsigned, usize_bits - 2);
+const IdSigned = std.meta.Int(.signed, usize_bits - 2);
+
+const State = packed struct(usize) {
+    id: Id,
+    state: DescState,
+};
+
+const Desc = struct {
+    state: std.atomic.Value(State),
+    blk_pos: BlkPos,
+};
+
+pub const Info = struct {
+    sequence: u64,
+    timestamp: u64,
+    length: u16,
+};
+
+const Entry = struct {
+    id: usize,
+    data: *[]u8,
+    data_size: u16,
+};
+
+const DataBlock = struct {
+    id: Id,
+    data: [0]u8,
+};
+
+pub const Reservation = struct {
+    irq_state: bool,
+    id: Id,
+    info: *Info,
+    buf: []u8,
+};
+
+pub const ReserveError = error{ NoSpace, ReservationFailed, ZeroSize };
+pub const ReadError = error{NotYetAvailable};
+
+// Logical positions are always aligned to `@sizeOf(usize)` (even numbers).
+// Thus, 1 (an odd number) is guaranteed to never collide with a real data block position.
+const lpos_no_data = 1;
+
+pub fn RingBuffer(data_size_bits: usize, avg_msg_bits: usize) type {
+    return struct {
+        const data_size = 1 << data_size_bits;
+        const desc_bits = data_size_bits - avg_msg_bits;
+        const desc_count = 1 << desc_bits;
+        const desc_mask = desc_count - 1;
+        const data_mask = data_size - 1;
+
+        descs: [desc_count]Desc,
+        infos: [desc_count]Info,
+        data: [data_size]u8,
+
+        /// ID of the newest descriptor.
+        head_id: std.atomic.Value(usize),
+        /// ID of the oldest descriptor.
+        tail_id: std.atomic.Value(usize),
+
+        /// Logical position of the next (not yet existing) data block.
+        head_lpos: std.atomic.Value(usize),
+
+        /// Beginning of the oldest data block.
+        tail_lpos: std.atomic.Value(usize),
+
+        const Self = @This();
+
+        // Returns the exact Id of the descriptor that occupied this physical slot one loop ago.
+        fn prev_wrap(id: Id) Id {
+            return id -% desc_count;
+        }
+
+        fn get_desc_state(id: Id, expected: State) DescState {
+            if (id != expected.id) {
+                return .Miss;
+            }
+
+            return expected.state;
+        }
+
+        fn to_block_size(size: usize) usize {
+            var sz = size;
+            sz += @sizeOf(DataBlock);
+            sz = std.mem.alignForward(usize, sz, @sizeOf(usize));
+            return sz;
+        }
+
+        fn to_desc(self: *Self, id: Id) *Desc {
+            return &self.descs[id & desc_mask];
+        }
+
+        fn to_info(self: *Self, id: Id) *Info {
+            return &self.infos[id & desc_mask];
+        }
+
+        fn to_block(self: *Self, lpos: usize) *DataBlock {
+            return @ptrCast(@alignCast(&self.data[lpos & data_mask]));
+        }
+
+        fn check_size(size: usize) bool {
+            // If the size is larger than half of the data buffer, it can never fit in the ring buffer.
+            return size <= data_size / 2;
+        }
+
+        fn need_more_space(lpos_current: usize, lpos_next: usize) bool {
+            // Unsigned wrapping arithmetic safely calculates distance between positions
+            // across overflow, checking if it exceeds the ring's physical capacity.
+            return lpos_next -% lpos_current -% 1 < data_size;
+        }
+
+        const ReadDescResult = struct {
+            state: DescState,
+            seq: u64 = 0,
+            desc: Desc = undefined,
+        };
+
+        fn read_desc(self: *Self, id: Id) ReadDescResult {
+            const desc = self.to_desc(id);
+            const info = self.to_info(id);
+            var state_var = desc.state.load(.acquire);
+            const state = get_desc_state(id, state_var);
+
+            var result: ReadDescResult = .{ .state = state };
+
+            if (state == .Miss or state == .Reserved) {
+                result.desc.state.store(state_var, .monotonic);
+                return result;
+            }
+
+            // Copy descriptor position and state block.
+            // Ordering is not critical for the payload copy itself.
+            result.desc.blk_pos = desc.blk_pos;
+            result.desc.state.store(state_var, .monotonic);
+
+            result.seq = info.sequence;
+
+            // Ensure the copy is finished before the state load.
+            rtl.barrier.rmb();
+
+            state_var = desc.state.load(.acquire);
+            result.state = get_desc_state(id, state_var);
+            result.desc.state.store(state_var, .monotonic);
+
+            return result;
+        }
+
+        fn read_desc_finalized(self: *Self, id: Id, seq: u64, desc_out: *Desc) !void {
+            const res = self.read_desc(id);
+            desc_out.* = res.desc;
+
+            if (res.state == .Miss or res.state == .Reserved or (res.seq != seq)) {
+                return error.Invalid;
+            }
+
+            if (res.state == .Free or (desc_out.blk_pos.begin == lpos_no_data))
+                return error.DataLost;
+        }
+
+        fn free_desc(self: *Self, id: Id) void {
+            var desc = self.to_desc(id);
+            const expected_state: State = .{ .state = .Published, .id = id };
+            const new_state: State = .{ .state = .Free, .id = id };
+
+            _ = desc.state.cmpxchgStrong(expected_state, new_state, .release, .monotonic);
+        }
+
+        fn free_data(self: *Self, begin: usize, end: usize) ?usize {
+            var desc: Desc = undefined;
+            var cur_begin = begin;
+
+            while (need_more_space(cur_begin, end)) {
+                const blk = self.to_block(cur_begin);
+
+                // Loading the ID here is purposely racey because we check if the descriptor is valid anyway.
+                const id: Id = @truncate(@atomicLoad(usize, @as(*usize, @ptrCast(&blk.id)), .monotonic));
+                const res = self.read_desc(id);
+                desc = res.desc;
+
+                switch (res.state) {
+                    .Miss, .Reserved => {
+                        // The block is still in use, we can't free it yet.
+                        return null;
+                    },
+                    .Published => {
+                        if (desc.blk_pos.begin != cur_begin) {
+                            // The descriptor does not point back to the data block, we must've lost a race.
+                            return null;
+                        }
+                        self.free_desc(id);
+                    },
+                    .Free => {
+                        if (desc.blk_pos.begin != cur_begin) {
+                            // The descriptor does not point back to the data block, we must've lost a race.
+                            return null;
+                        }
+                    },
+                }
+
+                // The block can be freed, move to the next one.
+                cur_begin = desc.blk_pos.end;
+            }
+
+            return cur_begin;
+        }
+
+        fn data_advance_tail(self: *Self, new_tail_lpos: usize) !void {
+            if (new_tail_lpos & 1 != 0) {
+                // No data.
+                return;
+            }
+
+            var cur_tail = self.tail_lpos.load(.monotonic);
+
+            // Loop until the tail is at or beyond the new tail position.
+            while (need_more_space(cur_tail, new_tail_lpos)) {
+                // Invalidate all data blocks before the new tail position.
+                if (self.free_data(cur_tail, new_tail_lpos)) |new_tail| {
+                    // Successfully freed some blocks, try advancing the tail.
+                    if (self.tail_lpos.cmpxchgWeak(cur_tail, new_tail, .release, .monotonic)) |real_cur_tail| {
+                        cur_tail = real_cur_tail;
+                        std.atomic.spinLoopHint();
+                    } else {
+                        break;
+                    }
+                } else {
+                    // Failed to free the data, which means we either lost a race or the descriptor is still in use.
+                    // Reload the tail position to try again.
+                    const new_tail = self.tail_lpos.load(.acquire);
+                    if (new_tail == cur_tail) {
+                        return error.ReservationFailed;
+                    }
+
+                    // Another CPU pushed the tail, try again.
+                    cur_tail = new_tail;
+                    std.atomic.spinLoopHint();
+                    continue;
+                }
+            }
+        }
+
+        // Try advancing the tail.
+        fn advance_tail(self: *Self, tail_id: Id) !void {
+            const res = self.read_desc(tail_id);
+            const tail_desc = res.desc;
+
+            switch (res.state) {
+                .Published => {
+                    // Free the descriptor.
+                    self.free_desc(tail_id);
+                },
+                .Reserved => {
+                    // The descriptor is still reserved, we can't do anything but fail the reservation.
+                    return error.ReservationFailed;
+                },
+                .Miss => {
+                    if (tail_desc.state.load(.monotonic).id == prev_wrap(tail_id)) {
+                        // This must mean that the descriptor is currently getting reserved by another writer.
+                        return error.ReservationFailed;
+                    }
+
+                    // Someone else has already advanced the tail, we are good.
+                    return;
+                },
+                .Free => {
+                    // The descriptor is already free, we can just advance the tail.
+                },
+            }
+
+            // Invalidate data blocks
+            try self.data_advance_tail(tail_desc.blk_pos.end);
+
+            // Read the next descriptor after tail_id, because the tail must always be in a finalized or free state.
+            const next_res = self.read_desc(tail_id +% 1);
+
+            if (next_res.state == .Published or next_res.state == .Free) {
+                // Advance the tail to the next descriptor.
+                // release: push all previous state changes before the tail update, guaranteeing the new tail is visible after all evictions.
+                _ = self.tail_id.cmpxchgStrong(@as(usize, tail_id), @as(usize, tail_id +% 1), .release, .monotonic);
+            } else {
+                // The next descriptor is not in the desired state.
+                // One possibility is that the tail was pushed by another CPU, in which case we are good.
+                const tail_id_cur = self.tail_id.load(.acquire);
+                if (tail_id_cur != tail_id) {
+                    // Another CPU pushed the tail, we are good.
+                    return;
+                }
+                return error.ReservationFailed;
+            }
+        }
+
+        // Reserve a new descriptor and make space for it if needed.
+        fn reserve_desc(self: *Self) !Id {
+            var head_id: Id = @truncate(self.head_id.load(.acquire));
+            var new_id: Id = undefined;
+            var prev_id: Id = undefined;
+
+            while (true) {
+                new_id = head_id +% 1;
+                prev_id = prev_wrap(new_id);
+
+                // Ordering: acquire pairs with release in advance_tail,
+                // guaranteeing we see the tail after any previous evictions.
+                const tail_id = self.tail_id.load(.acquire);
+
+                if (prev_id == tail_id) {
+                    // Wrapped back to the tail, need to make space by advancing it.
+                    try self.advance_tail(prev_id);
+                }
+
+                // Ordering: acq_rel to claim head and publish, acquire on failure to retry.
+                if (self.head_id.cmpxchgWeak(head_id, new_id, .acq_rel, .acquire)) |val| {
+                    head_id = @truncate(val);
+                    std.atomic.spinLoopHint();
+                } else {
+                    break;
+                }
+            }
+
+            const desc = self.to_desc(new_id);
+
+            // Ordering: monotonic is safe here because acq_rel on head_id published our claim.
+            const prev_state = desc.state.load(.monotonic);
+            const raw: usize = @bitCast(prev_state);
+
+            if (raw != 0 and get_desc_state(prev_id, prev_state) != .Free) {
+                return error.ReservationFailed;
+            }
+
+            // Ordering: release ensures all setup is published before making the descriptor reserved.
+            if (desc.state.cmpxchgStrong(prev_state, .{ .state = .Reserved, .id = new_id }, .release, .monotonic) != null) {
+                return error.ReservationFailed;
+            }
+
+            return new_id;
+        }
+
+        // Check if a block crosses the buffer boundary.
+        // `lpos` encodes a "generation" in its upper bits to track wrapping.
+        fn data_wraps(begin: usize, end: usize) bool {
+            return (begin >> data_size_bits) != ((end -% 1) >> data_size_bits);
+        }
+
+        fn get_next_lpos(lpos: usize, size: usize) usize {
+            const begin_lpos = lpos;
+            const next_lpos = lpos +% size;
+
+            // We never physically split a payload across the ring boundary.
+            // If it would cross the boundary, we advance the `lpos` to the next generation boundary
+            // so the data can sit contiguously at physical offset 0.
+            if (data_wraps(begin_lpos, next_lpos)) {
+                return ((next_lpos >> data_size_bits) << data_size_bits) + size;
+            }
+
+            return next_lpos;
+        }
+
+        fn alloc_data(self: *Self, size: usize, id: Id, out_position: *BlkPos) ReserveError![]u8 {
+            const blk_size = to_block_size(size);
+            var head_lpos = self.head_lpos.load(.monotonic);
+            var new_head_lpos = head_lpos;
+
+            if (size == 0) {
+                out_position.begin = lpos_no_data;
+                out_position.end = lpos_no_data;
+                return error.ZeroSize;
+            }
+
+            while (true) {
+                new_head_lpos = get_next_lpos(head_lpos, blk_size);
+
+                // Make room if needed, try advancing the tail until we have enough space.
+                self.data_advance_tail(new_head_lpos -% data_size) catch |e| {
+                    out_position.begin = lpos_no_data;
+                    out_position.end = lpos_no_data;
+                    return e;
+                };
+
+                // Try to claim the new head position.
+                if (self.head_lpos.cmpxchgWeak(head_lpos, new_head_lpos, .acq_rel, .acquire)) |real_head_lpos| {
+                    head_lpos = real_head_lpos;
+                    std.atomic.spinLoopHint();
+                } else {
+                    break;
+                }
+            }
+
+            var blk = self.to_block(head_lpos);
+            blk.id = id;
+
+            if (data_wraps(head_lpos, new_head_lpos)) {
+                // Wrapping blocks store their data at the beginning
+                blk = self.to_block(0);
+                blk.id = id;
+            }
+
+            out_position.begin = head_lpos;
+            out_position.end = new_head_lpos;
+
+            return @as([*]u8, @ptrCast(&blk.data))[0..size];
+        }
+
+        fn read_internal(self: *Self, seq: u64, buf: ?[]u8) !Info {
+            const rdesc = self.to_desc(@truncate(seq));
+            const info = self.to_info(@truncate(seq));
+            var desc: Desc = undefined;
+
+            const id = rdesc.state.load(.monotonic).id;
+
+            try self.read_desc_finalized(id, seq, &desc);
+
+            const result_info = info.*;
+
+            if (buf) |b| {
+                if (desc.blk_pos.begin == lpos_no_data) {
+                    return error.DataLost;
+                }
+
+                var blk = self.to_block(desc.blk_pos.begin);
+                if (data_wraps(desc.blk_pos.begin, desc.blk_pos.end)) {
+                    blk = self.to_block(0);
+                }
+
+                const data_ptr: [*]u8 = @ptrCast(&blk.data);
+                const len = @min(b.len, result_info.length);
+                @memcpy(b[0..len], data_ptr[0..len]);
+            }
+
+            try self.read_desc_finalized(id, seq, &desc);
+            return result_info;
+        }
+
+        /// Initialize and return a new ring buffer instance.
+        pub fn init() Self {
+            const dummy_id: Id = 0 -% @as(Id, desc_count + 1);
+            const blk0: usize = -%@as(usize, data_size);
+
+            var self: Self = std.mem.zeroes(Self);
+
+            self.descs[desc_count - 1].state.store(.{ .state = .Free, .id = dummy_id }, .monotonic);
+            self.descs[desc_count - 1].blk_pos = .{ .begin = lpos_no_data, .end = lpos_no_data };
+
+            self.head_id.store(dummy_id, .monotonic);
+            self.tail_id.store(dummy_id, .monotonic);
+
+            self.infos[0].sequence = -%(@as(u64, desc_count));
+
+            self.head_lpos.store(blk0, .monotonic);
+            self.tail_lpos.store(blk0, .monotonic);
+
+            return self;
+        }
+
+        /// Reserve space for a record and return a reservation.
+        pub fn reserve(self: *Self, size: usize) ReserveError!Reservation {
+            if (!check_size(size)) {
+                return error.NoSpace;
+            }
+
+            var reservation: Reservation = undefined;
+
+            reservation.irq_state = ki.impl.disable_interrupts();
+
+            const id = try self.reserve_desc();
+
+            const desc = self.to_desc(id);
+            var info = self.to_info(id);
+            const seq = info.sequence;
+
+            reservation.id = id;
+
+            if (seq == 0 and (id & desc_mask) != 0) {
+                // Bootstrap sequence number for initial descriptors.
+                info.sequence = @as(u64, id & desc_mask);
+            } else {
+                // Advance sequence by ring size for subsequent iterations.
+                info.sequence = seq +% desc_count;
+            }
+
+            reservation.buf = self.alloc_data(size, id, &desc.blk_pos) catch |e| {
+                // On failure, publish the descriptor as-is to clean up the reservation.
+                self.publish(reservation);
+                return e;
+            };
+
+            reservation.info = info;
+
+            return reservation;
+        }
+
+        /// Publish a previously reserved record.
+        pub fn publish(self: *Self, res: Reservation) void {
+            const desc = self.to_desc(res.id);
+
+            _ = desc.state.cmpxchgStrong(.{ .state = .Reserved, .id = res.id }, .{ .state = .Published, .id = res.id }, .release, .monotonic);
+
+            // Restore IRQ state after publishing the new entry.
+            ki.impl.restore_interrupts(res.irq_state);
+        }
+
+        /// Return the first readable sequence currently retained by the ring.
+        pub fn first_seq(self: *Self) u64 {
+            var seq: u64 = undefined;
+
+            while (true) {
+                const id: Id = @truncate(self.tail_id.load(.acquire));
+                const res = self.read_desc(id);
+                seq = res.seq;
+
+                if (res.state == .Published or res.state == .Free) break;
+                std.atomic.spinLoopHint();
+            }
+
+            return seq;
+        }
+
+        /// Read a record at `seq` into `buf`.
+        pub fn read(self: *Self, seq: u64, buf: ?[]u8) ReadError!Info {
+            var cur_seq = seq;
+
+            while (true) {
+                return self.read_internal(cur_seq, buf) catch |err| switch (err) {
+                    error.Invalid => {
+                        const first = self.first_seq();
+                        if (cur_seq < first) {
+                            // Behind the tail, catch up and try again.
+                            cur_seq = first;
+                            continue;
+                        }
+                        return error.NotYetAvailable;
+                    },
+                    error.DataLost => {
+                        // Record at cur_seq was overwritten, skip forward.
+                        const first = self.first_seq();
+                        if (cur_seq < first)
+                            cur_seq = first
+                        else
+                            cur_seq += 1;
+                        continue;
+                    },
+                };
+            }
+        }
+    };
+}

src/kern/panic.zig

@@ -5,15 +5,8 @@ const ki = ke.private;
 
 /// Panics!
 pub fn panic(comptime fmt: []const u8, args: anytype) noreturn {
-    ki.log.debug_lock.acquire_no_ipl();
+    // FIXME: this is probably broken.
+    std.log.err("KERNEL PANIC: " ++ fmt, args);
 
-    var buf: [1024]u8 = undefined;
-
-    const str = std.fmt.bufPrint(&buf, fmt, args) catch unreachable;
-
-    ki.log.print("Panic!\n");
-    ki.log.print(str);
-
-    ki.log.debug_lock.release_no_ipl();
     while (true) {}
 }

src/kern/private.zig

@@ -29,3 +29,4 @@ pub const timecounter = @import("timecounter.zig");
 pub const sched = @import("sched.zig");
 pub const wait = @import("wait.zig");
 pub const timer = @import("timer.zig");
+pub const log_ring = @import("log_ring.zig");

src/kern/root.zig

@@ -14,7 +14,6 @@ pub const TimeCounter = p.timecounter.TimeCounter;
 pub const Timer = p.timer.Timer;
 
 // === Exported Interfaces ===
-pub const log = p.log.log;
 pub const panic = p.panic.panic;
 
 pub const ipl = struct {
@@ -52,6 +51,13 @@ pub const wait = struct {
     pub const wait_any = p.wait.wait_any;
 };
 
+pub const log = struct {
+    pub const register_console = p.log.register_console;
+    pub const log = p.log.log;
+};
+
+pub const Console = p.log.Console;
+
 pub const Cpu = p.cpu.Cpu;
 pub const CpuLocal = p.cpu.CpuLocal;
 pub const CpuMask = p.cpu.CpuMask;

src/platform/um/impl.zig

@@ -39,8 +39,8 @@ pub threadlocal var my_cpu: *ke.Cpu = undefined;
 
 pub fn devices_init() void {}
 
-pub fn debug_write(char: u8) void {
+fn debug_write(_: *ke.Console, buf: []const u8) void {
-    std.debug.print("{c}", .{char});
+    _ = linux.write(1, buf.ptr, @intCast(buf.len));
 }
 
 pub const arm_timer = timer.arm_timer;
@@ -60,6 +60,12 @@ const bootstrap_offsets = [1]usize{0};
 
 pub var cpu_offsets: []usize = undefined;
 
+var early_console: ke.Console = .{
+    .write = debug_write,
+    .last_seq = 0,
+    .link = undefined,
+};
+
 pub fn early_init() linksection(b.init) void {
     // Ensure we can use per-cpu data.
     cpu_offsets = @constCast(&bootstrap_offsets);
@@ -100,6 +106,8 @@ pub fn early_init() linksection(b.init) void {
     b.kernel_heap_base = heap_base;
 
     std.log.info("um: kernel heap lives at {X}", .{heap_base});
+
+    ke.log.register_console(&early_console);
 }
 
 fn sigusr1_handler(_: posix.SIG, _: *const posix.siginfo_t, _: ?*anyopaque) callconv(.c) void {

src/root.zig

@@ -38,5 +38,5 @@ pub fn tib(comptime N: u32) usize {
 }
 
 pub const std_options = std.Options{
-    .logFn = ke.log,
+    .logFn = ke.log.log,
 };