The power button, in ring 3: SCI bound, fixed event published (M21.1)

The kernel publishes the FADT as one more acpi-tables memory resource
(tagged by its intact FACP header — the AML blobs are header-stripped);
the acpi service reads the PM1 event/control and GPE register ports from
that copy, so the kernel's own FADT parse is untouched. A power-protocol
module (ServiceId.power = 5, domain-named so an ARM PSCI service can serve
the same id) carries subscribe / shutdown / events.

The acpi service converts to runtime.service.run — device discovery, the
.power protocol, and the SCI notification all fold into one loop. At
startup it enables ACPI mode if SCI_EN is clear (the SMI dance), binds the
SCI (found as the node's len-1 irq resource, distinct from the broad
window), and sets PWRBTN_EN. On the SCI it reads PM1_STS, clears
PWRBTN_STS write-1, logs the press, publishes power_button to
subscribers, and always acks. The power-button scenario proves it with a
real QMP system_powerdown injected mid-run through the M21.0 channel.

system/devices/acpi.zig

@@ -157,6 +157,13 @@ pub var namespace: ?aml.Namespace = null;
 /// Physical address of the DSDT the FADT points at, or 0.
 pub var dsdt_physical: u64 = 0;
 
+/// The FADT itself (physical + length), published on the acpi-tables node so
+/// the ring-3 acpi service can read the PM1 event and GPE blocks it needs for
+/// the event side (docs/m21-plan.md decision 3). Distinguished from the AML
+/// blob resources by its intact "FACP" header — the blobs are header-stripped.
+var fadt_physical: u64 = 0;
+var fadt_length: u64 = 0;
+
 // AML blocks (DSDT + any SSDTs) collected during the table walk, as physical
 // address + length of each table's post-header bytecode. Scanned after the walk
 // for the sleep-state (`_Sx`) packages.
@@ -373,6 +380,8 @@ pub fn discover(rsdp_physical: u64, memory_regions: []const boot_handoff.MemoryR
 
     // Start clean so a re-run doesn't accumulate stale state.
     power_information = .{};
+    fadt_physical = 0;
+    fadt_length = 0;
     platform_information = .{};
     aml_stats = .{};
     namespace = null;
@@ -447,6 +456,9 @@ fn publishAcpiTablesNode(device_tree: *DeviceTree) !void {
     // SCI (recorded first, len 1) stays distinct so M21 can pick it out.
     if (power_information.sci_interrupt != 0) _ = node.addResource(.irq, power_information.sci_interrupt, 1);
     _ = node.addResource(.irq, 0, 256);
+    // The FADT rides along (M21): the service reads the PM1 event / GPE blocks
+    // from its own copy, telling it apart from the AML blobs by signature.
+    if (fadt_physical != 0) _ = node.addResource(.memory, fadt_physical, fadt_length);
 }
 
 /// The number of Device objects in the namespace built during discovery, or 0.
@@ -482,6 +494,8 @@ fn handleTable(device_tree: *DeviceTree, hal: Hal, sdt_physical: u64) !void {
     } else if (std.mem.eql(u8, &sig, &HPET)) {
         try parseHpet(device_tree, hal, header);
     } else if (std.mem.eql(u8, &sig, &FACP)) {
+        fadt_physical = sdt_physical;
+        fadt_length = header.length;
         parseFadt(header);
     } else if (std.mem.eql(u8, &sig, &SPCR)) {
         parseSpcr(header);