bzimage: Improve initrd memory allocation

Simplified checks for address/size out of range but also a new check to
limit to identity mapped 4GiB region.

The potential address is now clamped to the max start address if that is
in the range. This fixes the issue with booting with more than 2GiB as
the Linux max is just under 2GiB and previously the potential address
would be at the top of the 3GiB boundary when running with more than
2GiB.

Tested with image boot with initrd with 1GiB and 4GiB RAM.

Fixes: #175

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

Author: rbradford

src/bzimage.rs

@@ -82,30 +82,50 @@ impl Kernel {
             0 => 0x37FF_FFFF,
             a => a as u64,
         };
+
+        // Limit to 4GiB identity mapped area
+        let initrd_addr_max = u64::min(initrd_addr_max, (4 << 30) - 1);
+
         let max_start = (initrd_addr_max + 1) - size;
 
-        let mut option_addr = None;
+        // Align address to 2MiB boundary as we use 2 MiB pages
+        let max_start = max_start & !((2 << 20) - 1);
+
+        let mut current_addr = None;
         for i in 0..self.0.num_entries() {
             let entry = self.0.entry(i);
             if entry.entry_type != E820Entry::RAM_TYPE {
                 continue;
             }
-            let addr = entry.addr + entry.size - size;
-            // Align address to 2MiB boundary as we use 2 MiB pages
-            let addr = addr & !((2 << 20) - 1);
-            // The ramdisk must fit in the region completely
-            if addr > max_start || addr < entry.addr {
+
+            // Disregard regions beyond the max
+            if entry.addr > max_start {
                 continue;
             }
-            // Use the largest address we can find
-            if let Some(load_addr) = option_addr {
-                if load_addr >= addr {
+
+            // Disregard regions that are too small
+            if size > entry.size {
+                continue;
+            }
+
+            // Place at the top of the region
+            let potential_addr = entry.addr + entry.size - size;
+
+            // Align address to 2MiB boundary as we use 2 MiB pages
+            let potential_addr = potential_addr & !((2 << 20) - 1);
+
+            // But clamp to the maximum start
+            let potential_addr = u64::min(potential_addr, max_start);
+
+            // Use the higest address we can find
+            if let Some(current_addr) = current_addr {
+                if current_addr >= potential_addr {
                     continue;
                 }
             }
-            option_addr = Some(addr)
+            current_addr = Some(potential_addr)
         }
-        option_addr
+        current_addr
     }
 
     pub fn load_initrd(&mut self, f: &mut dyn Read) -> Result<(), Error> {