VirtIO-blk

MMIO legacy (version 1) transport backing the Debian rootfs.

Layout

Transport

• Legacy MMIO v1 — matches Linux's virtio_mmio driver without needing the modern VIRTIO_F_VERSION_1 path.
• Split virtqueue, 128 entries.
• Synchronous DMA processing — process_dma reads descriptors from guest RAM, dispatches the request, writes status, rings the used ring.

DmaCtx

Bus-mediated guest-memory accessor — the only bridge between VirtIO code and guest RAM:

#![allow(unused)]
fn main() {
impl<'a> DmaCtx<'a> {
    pub fn read_bytes(&mut self, gpa: u64, buf: &mut [u8]) -> XResult<()>;
    pub fn write_bytes(&mut self, gpa: u64, buf: &[u8]) -> XResult<()>;
    pub fn read_le<T: LeBytes>(&mut self, gpa: u64) -> XResult<T>;
    pub fn write_le<T: LeBytes>(&mut self, gpa: u64, v: T) -> XResult<()>;
}
}

The LeBytes trait is the type-safe layer — no unsafe aliasing of guest memory, just bounded &mut [u8] views through the Bus.

BlkStorage

Separated from the transport state so Rust's borrow checker can split them:

#![allow(unused)]
fn main() {
struct VirtioBlk {
    transport: TransportState,   // queue pointers, device status
    storage:   BlkStorage,        // backing snapshot
}
}

process_dma borrows &mut TransportState + &mut BlkStorage disjointly — no interior mutability, no runtime borrow tracking.

Two-tier reset

• Transport reset — QueueReady goes to 0; QueueSel is cleared; disk contents preserved.
• Emulator hard reset — via the test finisher; restores the disk to the snapshot recorded at load.

Debian image

resource/xemu-debian.img — 4 GiB ext4 filesystem with Debian 13 Trixie pre-installed. Build system downloads it on first make debian.