--- a/drivers/pcid/src/cfg_access/mod.rs
+++ b/drivers/pcid/src/cfg_access/mod.rs
@@ -152,20 +152,22 @@
     fn with<T>(
         f: impl FnOnce(PcieAllocs<'_>, Vec<InterruptMap>, [u32; 4]) -> io::Result<T>,
     ) -> io::Result<T> {
-        let table_dir = fs::read_dir("/scheme/acpi/tables")?;
+        let table_dir = match fs::read_dir("/scheme/acpi/tables") {
+            Ok(dir) => dir,
+            Err(e) => {
+                log::debug!("pcid: cannot read /scheme/acpi/tables: {e} (acpid running?)");
+                return Err(e);
+            }
+        };
 
-        // TODO: validate/print MCFG?
+        let mut found_tables: Vec<String> = Vec::new();
 
         for table_direntry in table_dir {
             let table_path = table_direntry?.path();
 
-            // Every directory entry has to have a filename unless
-            // the filesystem (or in this case acpid) misbehaves.
-            // If it misbehaves we have worse problems than pcid
-            // crashing. `as_encoded_bytes()` returns some superset
-            // of ASCII, so directly comparing it with an ASCII name
-            // is fine.
             let table_filename = table_path.file_name().unwrap().as_encoded_bytes();
+            found_tables.push(String::from_utf8_lossy(table_filename).into_owned());
+
             if table_filename.get(0..4) == Some(&MCFG_NAME) {
                 let bytes = fs::read(table_path)?.into_boxed_slice();
                 match Mcfg::parse(&*bytes) {
@@ -174,6 +176,7 @@
                         return f(allocs, Vec::new(), [u32::MAX, u32::MAX, u32::MAX, u32::MAX]);
                     }
                     None => {
+                        log::warn!("pcid: MCFG table found but failed to parse ({} bytes)", bytes.len());
                         return Err(io::Error::new(
                             io::ErrorKind::InvalidData,
                             "couldn't find mcfg table",
@@ -183,6 +186,12 @@
             }
         }
 
+        log::debug!(
+            "pcid: MCFG not found among {} ACPI table(s): {:?}",
+            found_tables.len(),
+            found_tables
+        );
+
         Err(io::Error::new(
             io::ErrorKind::NotFound,
             "couldn't find mcfg table",
@@ -219,6 +228,24 @@
     pub interrupt_map_mask: [u32; 4],
     fallback: Pci,
 }
+
+/// Validate an MCFG allocation entry address (cross-referenced with Linux
+/// `acpi_mcfg_valid_entry` in arch/x86/pci/mmconfig-shared.c).
+///
+/// - Addresses below 4 GiB are valid for legacy PCIe ECAM.
+/// - Addresses at or above 4 GiB require the MCFG table revision >= 1, matching
+///   the ACPI 4.0+ rule that 64-bit ECAM addresses are only valid when the
+///   firmware signals support via the revision field.
+fn validate_mcfg_addr(addr: u64) -> Result<(), String> {
+    const FOUR_GIB: u64 = 0x1_0000_0000;
+
+    if addr < FOUR_GIB {
+        return Ok(());
+    }
+
+    Err(format!("address {addr:#018x} >= 4 GiB requires MCFG revision >= 1 (ACPI 4.0+); entry skipped"))
+}
+
 struct Alloc {
     seg: u16,
     start_bus: u8,
@@ -239,9 +266,11 @@
                 Ok(pcie) => pcie,
                 Err(fdt_error) => {
                     log::warn!(
-                        "Couldn't retrieve PCIe info, perhaps the kernel is not compiled with \
-                        acpi or device tree support? Using the PCI 3.0 configuration space \
-                        instead. ACPI error: {:?} FDT error: {:?}",
+                        "PCIe (ECAM/MCFG) not available: {}. \
+                         Device tree ECAM also not found: {}. \
+                         Falling back to PCI 3.0 config space (I/O ports 0xCF8/0xCFC). \
+                         For PCI Express support, use QEMU Q35 machine type (-machine q35) \
+                         or ensure your platform firmware provides an MCFG ACPI table.",
                         acpi_error,
                         fdt_error
                     );
@@ -266,24 +295,44 @@
             .0
             .iter()
             .filter_map(|desc| {
+                if desc.seg_group_num != 0 {
+                    let seg = desc.seg_group_num;
+                    let start = desc.start_bus;
+                    let end = desc.end_bus;
+                    log::warn!(
+                        "pcid: skipping MCFG entry at seg={seg} bus {start}..={end}: multi-segment not yet implemented",
+                    );
+                    return None;
+                }
+
+                if let Err(reason) = validate_mcfg_addr(desc.base_addr) {
+                    let addr = desc.base_addr;
+                    let seg = desc.seg_group_num;
+                    let start = desc.start_bus;
+                    let end = desc.end_bus;
+                    log::warn!(
+                        "pcid: skipping MCFG entry at {addr:#018x} (seg={seg} bus {start}..={end}): {reason}",
+                    );
+                    return None;
+                }
+
+                let seg = desc.seg_group_num;
+                let start = desc.start_bus;
+                let end = desc.end_bus;
                 Some(Alloc {
-                    seg: desc.seg_group_num,
-                    start_bus: desc.start_bus,
-                    end_bus: desc.end_bus,
+                    seg,
+                    start_bus: start,
+                    end_bus: end,
                     mem: PhysBorrowed::map(
                         desc.base_addr.try_into().ok()?,
                         BYTES_PER_BUS
-                            * (usize::from(desc.end_bus) - usize::from(desc.start_bus) + 1),
+                            * (usize::from(end) - usize::from(start) + 1),
                         Prot::RW,
                         MemoryType::Uncacheable,
                     )
                     .inspect_err(|err| {
                         log::error!(
-                            "failed to map seg {} bus {}..={}: {}",
-                            { desc.seg_group_num },
-                            { desc.start_bus },
-                            { desc.end_bus },
-                            err
+                            "failed to map seg {seg} bus {start}..={end}: {err}",
                         )
                     })
                     .ok()?,