USB2/1.1 (EHCI/UHCI) drivers + real-hardware bring-up hardening

- Add polled EHCI (USB2) and UHCI (USB1.1) host-controller drivers with
  HID boot mouse support, wired into cursor polling and the 'usb' shell cmd
- Add USB Legacy ownership handoff (claim_usb_legacy) plus config_write_byte
- Harden boot: verify the PIT/timer ticks before shell entry, EOI stray
  interrupts in the LAPIC to stop interrupt storms, bound PS/2 controller
  waits, and INTx-disable AHCI (polled driver)
- Add boot stage markers, klog screen echo, and QR lock; scheduler
  mark_shell_running; NO_USB stopgap to keep BIOS USB-Legacy input alive
- alloc_pages_above for DMA buffers that must avoid low conventional memory
- Expand DEBUG_NOTES with EHCI/QEMU bring-up findings

1 files changed, 5 insertions, 2 deletions

diff --git a/rsnix/src/ahci.rs b/rsnix/src/ahci.rs
index a77b341..543bb21 100644
--- a/rsnix/src/ahci.rs
+++ b/rsnix/src/ahci.rs
@@ -230,9 +230,12 @@ pub fn init() -> bool {
     let Some((dev, func)) = find_ahci() else {
         return false;
     };
-    // Enable memory space + bus mastering so MMIO and DMA work.
+    // Enable memory space + bus mastering so MMIO and DMA work, plus INTx
+    // disable (bit 10): the driver polls command completion (no ISR, no
+    // MSI), so the HBA must not assert INTx — a stray SATA interrupt would
+    // re-fire forever on real hardware.
     let cmd_reg = pci::config_read(0, dev, func, 0x04);
-    pci::config_write(0, dev, func, 0x04, cmd_reg | 0x7);
+    pci::config_write(0, dev, func, 0x04, cmd_reg | 0x7 | 0x400);
 
     let Some(abar) = pci::bar64(0, dev, func, 5) else {
         return false;