Skip remaining PCI devices when device 0 is absent on a bus

Standard PCI enumeration optimization: if device 0 function 0 on a bus
returns 0xFFFF, no devices exist on that bus. This reduces the scan from
32 buses × 32 devices = 1024 probes to ~63 probes on uhyve (which has
a single device at bus 0 device 0).

PCI legacy scan drops from ~13 ms to ~0.8 ms, cutting total boot time
from ~18 ms to ~5.5 ms.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jbreitbart

src/arch/x86_64/kernel/pci.rs

@@ -92,12 +92,14 @@ pub(crate) fn init() {
 		return;
 	}
 
+	boot_timestamp!("pci: legacy scan start");
 	// For Hermit, we currently limit scanning to the first 32 buses.
 	const PCI_MAX_BUS_NUMBER: u8 = 32;
 	scan_bus(
 		0..PCI_MAX_BUS_NUMBER,
 		PciConfigRegion::Pci(LegacyPciConfigRegion::new()),
 	);
+	boot_timestamp!("pci: legacy scan end");
 	info!("Initialized PCI");
 }
 
@@ -114,10 +116,16 @@ fn scan_bus(bus_range: impl IntoIterator<Item = u8> + Debug, pci_config: PciConf
 			let header = PciHeader::new(pci_address);
 
 			let (device_id, vendor_id) = header.id(pci_config);
-			if device_id != u16::MAX && vendor_id != u16::MAX {
-				let device = PciDevice::new(pci_address, pci_config);
-				PCI_DEVICES.with(|pci_devices| pci_devices.unwrap().push(device));
+			if device_id == u16::MAX || vendor_id == u16::MAX {
+				// If device 0 on this bus is absent, no devices exist on
+				// this bus — skip the remaining device slots.
+				if device == 0 {
+					break;
+				}
+				continue;
 			}
+			let device = PciDevice::new(pci_address, pci_config);
+			PCI_DEVICES.with(|pci_devices| pci_devices.unwrap().push(device));
 		}
 	}
 }