agent: PCI slot type

Add a Rust type for representing a PCI slot on a single bus.  This is
essentially just an integer from 0..31 (inclusive), but includes the
code for converting from integers with appropriate validation and
formatting back to a string.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/agent/src/main.rs

@@ -49,6 +49,7 @@ mod mount;
 mod namespace;
 mod netlink;
 mod network;
+mod pci;
 pub mod random;
 mod sandbox;
 #[cfg(test)]

src/agent/src/pci.rs

@@ -0,0 +1,88 @@
+// Copyright Red Hat.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+use std::convert::TryInto;
+use std::fmt;
+use std::str::FromStr;
+
+use anyhow::anyhow;
+
+// The PCI spec reserves 5 bits for slot number (a.k.a. device
+// number), giving slots 0..31
+const SLOT_BITS: u8 = 5;
+const SLOT_MAX: u8 = (1 << SLOT_BITS) - 1;
+
+// Represents a PCI function's slot number (a.k.a. device number),
+// giving its location on a single bus
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct Slot(u8);
+
+impl Slot {
+    pub fn new<T: TryInto<u8> + fmt::Display + Copy>(v: T) -> anyhow::Result<Self> {
+        if let Ok(v8) = v.try_into() {
+            if v8 <= SLOT_MAX {
+                return Ok(Slot(v8));
+            }
+        }
+        Err(anyhow!(
+            "PCI slot {} should be in range [0..{:#x}]",
+            v,
+            SLOT_MAX
+        ))
+    }
+}
+
+impl FromStr for Slot {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> anyhow::Result<Self> {
+        let v = isize::from_str_radix(s, 16)?;
+        Slot::new(v)
+    }
+}
+
+impl fmt::Display for Slot {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{:02x}", self.0)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::pci::Slot;
+    use std::str::FromStr;
+
+    #[test]
+    fn test_slot() {
+        // Valid slots
+        let slot = Slot::new(0x00).unwrap();
+        assert_eq!(format!("{}", slot), "00");
+
+        let slot = Slot::from_str("00").unwrap();
+        assert_eq!(format!("{}", slot), "00");
+
+        let slot = Slot::new(31).unwrap();
+        let slot2 = Slot::from_str("1f").unwrap();
+        assert_eq!(slot, slot2);
+
+        // Bad slots
+        let slot = Slot::new(-1);
+        assert!(slot.is_err());
+
+        let slot = Slot::new(32);
+        assert!(slot.is_err());
+
+        let slot = Slot::from_str("20");
+        assert!(slot.is_err());
+
+        let slot = Slot::from_str("xy");
+        assert!(slot.is_err());
+
+        let slot = Slot::from_str("00/");
+        assert!(slot.is_err());
+
+        let slot = Slot::from_str("");
+        assert!(slot.is_err());
+    }
+}