skip reads if the data is already buffered

read_exact_at is often called with a bunch of offsets in the same
general area. In a lot of cases, the data needed is still in the
buffer from the last call. Let's make use of that by remembering the
last sector read and skip reading the same sector again.

Author: Freax13

bios/stage-2/src/disk.rs

@@ -5,6 +5,9 @@ pub struct DiskAccess {
     pub disk_number: u16,
     pub base_offset: u64,
     pub current_offset: u64,
+
+    /// The offset of the sector read in the last call to read_exact_at.
+    pub last_read_exact_at_offset: Option<u64>,
 }
 
 impl Read for DiskAccess {
@@ -19,7 +22,15 @@ impl Read for DiskAccess {
         let buf = unsafe { &mut TMP_BUF };
         assert!(current_sector_offset + len <= buf.buffer.len());
 
-        self.read_exact_into(buf.buffer.len(), buf);
+        // Don't read the sector if we already read the same sector in the last
+        // call.
+        if self
+            .last_read_exact_at_offset
+            .is_none_or(|offset| offset != self.current_offset)
+        {
+            self.last_read_exact_at_offset = Some(self.current_offset);
+            self.read_exact_into(buf.buffer.len(), buf);
+        }
 
         &buf.buffer[current_sector_offset..][..len]
     }

bios/stage-2/src/main.rs

@@ -83,6 +83,7 @@ fn start(disk_number: u16, partition_table_start: *const u8) -> ! {
         disk_number,
         base_offset: u64::from(fat_partition.logical_block_address) * 512,
         current_offset: 0,
+        last_read_exact_at_offset: None,
     };
 
     let mut fs = fat::FileSystem::parse(disk.clone());