fix: align MMIO mappings to page boundaries for VirtIO capabilities

VirtIO capability structures can start at non-page-aligned offsets within\nBARs. The kernel physmap requires page-aligned addresses. Align the physical\naddress down, adjust the size, and apply the page offset to the returned\npointer. Track the original map pointer/size separately for correct unmapping.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

local/recipes/drivers/redox-driver-sys/source/src/memory.rs

@@ -81,6 +81,8 @@ fn ensure_memory_root() -> Result<libredox::Fd> {
 pub struct MmioRegion {
     ptr: *mut u8,
     size: usize,
+    map_ptr: *mut u8,
+    map_size: usize,
 }
 
 impl MmioRegion {
@@ -89,7 +91,13 @@ impl MmioRegion {
             return Err(DriverError::InvalidAddress(phys_addr));
         }
 
-        let aligned_size = size.next_multiple_of(PAGE_SIZE);
+        // Align physical address down to page boundary for physmap compatibility.
+        // The kernel requires page-aligned addresses; VirtIO capabilities can
+        // start at non-page-aligned offsets within BARs.
+        let page_offset = (phys_addr as usize) % PAGE_SIZE;
+        let aligned_phys = phys_addr - page_offset as u64;
+        let adjusted_size = size + page_offset;
+        let aligned_size = adjusted_size.next_multiple_of(PAGE_SIZE);
         let path = format!("physical@{}", cache.suffix());
 
         let mode = if prot.contains(MmioProt::READ | MmioProt::WRITE) {
@@ -112,7 +120,7 @@ impl MmioRegion {
         let mem_fd = root_fd.openat(&path, (O_CLOEXEC | mode) as i32, 0)?;
 
         let map = Map {
-            offset: phys_addr as usize,
+            offset: aligned_phys as usize,
             size: aligned_size,
             flags: mmap_prot | redox_syscall::MapFlags::from_bits_truncate(MAP_SHARED.bits()),
             address: 0,
@@ -127,8 +135,10 @@ impl MmioRegion {
         })?;
 
         Ok(Self {
-            ptr: ptr as *mut u8,
-            size: aligned_size,
+            ptr: unsafe { (ptr as *mut u8).add(page_offset) },
+            size,
+            map_ptr: ptr as *mut u8,
+            map_size: aligned_size,
         })
     }
 
@@ -290,8 +300,8 @@ impl MmioRegion {
 
 impl Drop for MmioRegion {
     fn drop(&mut self) {
-        if !self.ptr.is_null() {
-            let _ = unsafe { libredox::call::munmap(self.ptr as *mut (), self.size) };
+        if !self.map_ptr.is_null() {
+            let _ = unsafe { libredox::call::munmap(self.map_ptr as *mut (), self.map_size) };
         }
     }
 }