feat(storage): ext4-only disk via mkfs.ext4 -d; drop FAT16; feature-gated mount (ro on unknown ro_compat); ext4 read spans blocks, delete clears inode, mkdir counts dir; virtio-blk writes; test-qemu e2fsck gate

crates/hw-virtio/src/block.rs

@@ -60,3 +60,37 @@ pub fn read_sector(
 
     true
 }
+
+/// Request types (virtio spec 5.2.6).
+const T_OUT: u32 = 1;
+/// Device configuration: the disk's size in 512-byte sectors (le64).
+pub const CONFIG_CAPACITY: usize = 0x100;
+
+/// Writes one sector from `buf`; true once the device reports success.
+pub fn write_sector(
+    mmio: &impl MmioAccess,
+    queue: &VirtioQueue,
+    dma: &DmaAddrs,
+    sector: u32,
+    buf: &[u8],
+) -> bool {
+    if buf.len() < SECTOR_SIZE { return false; }
+
+    unsafe {
+        core::ptr::copy_nonoverlapping(buf.as_ptr(), dma.data_va.raw() as *mut u8, SECTOR_SIZE);
+        let hdr = dma.header_va.raw() as *mut BlockReqHeader;
+        core::ptr::write_volatile(&raw mut (*hdr).req_type, T_OUT);
+        core::ptr::write_volatile(&raw mut (*hdr).reserved, 0);
+        core::ptr::write_volatile(&raw mut (*hdr).sector, sector as u64);
+        core::ptr::write_volatile(dma.status_va.raw() as *mut u8, 0xFF);
+    }
+
+    // The device reads the data this time; only the status is written back.
+    let descs = [
+        Descriptor { addr: dma.header_pa.raw() as u64, len: 16, writable: false },
+        Descriptor { addr: dma.data_pa.raw() as u64, len: SECTOR_SIZE as u32, writable: false },
+        Descriptor { addr: dma.status_pa.raw() as u64, len: 1, writable: true },
+    ];
+    if !queue.submit(mmio, &descs) { return false; }
+    unsafe { core::ptr::read_volatile(dma.status_va.raw() as *const u8) == 0 }
+}