vmm: acpi: provide FADT PM1a event/control blocks for nested Hyper-V

A Windows guest that launches nested Hyper-V (for example to run WSL2)
fails to start its hypervisor on cloud-hypervisor's HW-reduced-ACPI
FADT. hvloader's hypervisor-launch path (0x18000f01c -> 0x180015628 ->
0x180015788) registers every legacy PM register block via 0x1800158dc
and rejects any block whose GAS address is 0 with status 8
(STATUS_INVALID_DEVICE_REQUEST). hvix64 then never launches and
HypervisorPresent stays False. The HW-reduced FADT leaves those blocks
zero.

Emit valid PM1a event/control blocks (I/O ports, lengths and X_GAS) in
the FADT. The HW-reduced guest OS ignores the legacy ports; only
hvloader's ACPI validation reads them.

These blocks are only useful to a guest that itself runs a hypervisor,
so emit them only when guest nesting is enabled (--cpu nested=on).

Signed-off-by: doge <me@crackerben.com>

Author: doge-rgb

vmm/src/acpi.rs

@@ -276,7 +276,11 @@ pub fn create_dsdt_table(
 
 const FACP_DSDT_OFFSET: usize = 140;
 
-fn create_facp_table(dsdt_offset: GuestAddress, device_manager: &DeviceManager) -> Sdt {
+fn create_facp_table(
+    dsdt_offset: GuestAddress,
+    device_manager: &DeviceManager,
+    nested: bool,
+) -> Sdt {
     trace_scoped!("create_facp_table");
 
     // Revision 6 of the ACPI FADT table is 276 bytes long
@@ -328,6 +332,28 @@ fn create_facp_table(dsdt_offset: GuestAddress, device_manager: &DeviceManager)
     // Hypervisor Vendor Identity
     facp.write_bytes(268, b"CLOUDHYP");
 
+    // PM1a event/control blocks for Windows nested Hyper-V's hvloader (which rejects a
+    // FADT with a zero PM1a GAS). Gated on `nested`; HW-reduced ACPI hides it from guests.
+    if nested {
+        const PM1A_EVT_PORT: u16 = 0x60c;
+        facp.write(56usize, PM1A_EVT_PORT as u32); // PM1a_EVT_BLK (0x38)
+        facp.write(88usize, 4u8); // PM1_EVT_LEN (0x58)
+        facp.write(148usize, 1u8); // X_PM1a_EVT_BLK GAS (0x94): AddressSpaceId=SystemIO
+        facp.write(149usize, 32u8); // RegisterBitWidth
+        facp.write(150usize, 0u8); // RegisterBitOffset
+        facp.write(151usize, 2u8); // AccessSize = word (16-bit)
+        facp.write(152usize, PM1A_EVT_PORT as u64); // Address
+
+        const PM1A_CNT_PORT: u16 = 0x610;
+        facp.write(64usize, PM1A_CNT_PORT as u32); // PM1a_CNT_BLK (0x40)
+        facp.write(89usize, 2u8); // PM1_CNT_LEN (0x59)
+        facp.write(172usize, 1u8); // X_PM1a_CNT_BLK GAS (0xac): AddressSpaceId=SystemIO
+        facp.write(173usize, 16u8); // RegisterBitWidth
+        facp.write(174usize, 0u8); // RegisterBitOffset
+        facp.write(175usize, 2u8); // AccessSize = word (16-bit)
+        facp.write(176usize, PM1A_CNT_PORT as u64); // Address
+    }
+
     facp.update_checksum();
 
     facp
@@ -857,7 +883,12 @@ fn create_acpi_tables_internal(
     tables_bytes.extend_from_slice(dsdt.as_slice());
 
     // FACP aka FADT
-    let facp = create_facp_table(dsdt_addr, device_manager);
+    // PM1a blocks are x86-only; there is nothing to emit on other arches.
+    #[cfg(target_arch = "x86_64")]
+    let nested = cpu_manager.nested();
+    #[cfg(not(target_arch = "x86_64"))]
+    let nested = false;
+    let facp = create_facp_table(dsdt_addr, device_manager, nested);
     let facp_addr = dsdt_addr.checked_add(dsdt.len() as u64).unwrap();
     tables_bytes.extend_from_slice(facp.as_slice());
     xsdt_table_pointers.push(facp_addr.0);
@@ -1132,7 +1163,8 @@ pub fn create_acpi_tables_tdx(
     )];
 
     // FACP aka FADT
-    tables.push(create_facp_table(GuestAddress(0), device_manager));
+    let nested = cpu_manager.nested();
+    tables.push(create_facp_table(GuestAddress(0), device_manager, nested));
 
     // MADT
     tables.push(cpu_manager.create_madt());

vmm/src/cpu.rs

@@ -1093,6 +1093,11 @@ impl CpuManager {
         Ok(())
     }
 
+    #[cfg(target_arch = "x86_64")]
+    pub fn nested(&self) -> bool {
+        self.config.nested
+    }
+
     /// Only create new vCPUs if there aren't any inactive ones to reuse
     fn create_vcpus(
         &mut self,