wake application processors to long mode

INIT-SIPI-SIPI plus a self-relocating real-mode trampoline.

src/kernel/pmm.zig

@@ -141,6 +141,24 @@ pub fn alloc() ?u64 {
     return null; // out of physical memory
 }
 
+/// Allocate one free frame whose physical address is below `limit`, or null if
+/// none is free down there. The AP trampoline needs this: an x86 STARTUP IPI vectors
+/// a waking core to physical `vector << 12`, and `vector` is a byte — so the
+/// trampoline must live under 1 MiB. A short linear scan of the low frames; only run
+/// a handful of times at boot, so it needn't be fast.
+pub fn allocBelow(limit: u64) ?u64 {
+    const cap = @min(total_frames, @as(usize, @intCast(limit / page_size)));
+    var f: usize = 1; // frame 0 stays reserved as the "none" address
+    while (f < cap) : (f += 1) {
+        if (!isUsed(f)) {
+            setUsed(f);
+            used_frames += 1;
+            return @as(u64, f) * page_size;
+        }
+    }
+    return null;
+}
+
 /// Return a frame obtained from alloc() to the pool. Bogus or double frees are
 /// ignored rather than corrupting the count.
 pub fn free(addr: u64) void {