VirtIO Block

VirtIO is a standardized interface for exposing virtual devices to a guest, typically under a hypervisor such as QEMU. The virtio,blk driver presents a VirtIO block device as a Zephyr disk: it is reachable through the Disk Access API and, through it, the File System API.

The driver is transport-agnostic and runs unchanged over both VirtIO transports Zephyr supports, PCI and MMIO (see VirtIO). An instance is created for every enabled virtio,blk devicetree node.

Driver design

The driver lives in drivers/disk/virtio_blk.c and is layered on top of the generic VirtIO API. A virtio,blk node is a child of a VirtIO transport device (DT_INST_PARENT), so the same code serves a PCI or an MMIO device without change.

Single in-flight request

The disk_access read/write API is synchronous and blocking. The driver also serializes callers with a mutex, so each device keeps exactly one request in flight: it adds a single descriptor chain to the request virtqueue, notifies the device, and blocks on a semaphore that the completion callback signals from interrupt context.

Because there is no request pipelining, the whole virtqueue is spent on a single request’s scatter-gather chain. The user-facing knob is the maximum number of data segments per request (CONFIG_DISK_VIRTIO_BLK_MAX_SEGMENTS); the driver sizes the virtqueue by rounding MAX_SEGMENTS + 2 up to a power of two, leaving room for the request header and the status byte.

Block size

The VirtIO block protocol always addresses the device in fixed 512-byte sectors and reports device capacity in those units. The logical block size exposed to disk_access may differ:

  • When the device offers VIRTIO_BLK_F_BLK_SIZE, the driver negotiates it and exposes the device-advertised blk_size.

  • Otherwise it falls back to CONFIG_DISK_VIRTIO_BLK_SECTOR_SIZE.

All addressing is converted to 512-byte VirtIO sectors internally. The block size must be a multiple of 512 and, under CONFIG_MMU, must not exceed the MMU page size; an unsupported value is rejected at initialization.

Zero-copy transfers

Caller buffers are handed to the device directly, without a bounce buffer. Under CONFIG_MMU a buffer may lie outside the kernel’s linear RAM mapping (for example the thread-stack work area f_mkfs() passes down) and need not be physically contiguous. The driver walks such a buffer page by page with arch_page_phys_get(), coalesces physically adjacent runs into scatter-gather segments, and, when a transfer is more fragmented than the segment budget allows, splits it across several device requests.

Feature negotiation

Besides VIRTIO_BLK_F_BLK_SIZE the driver negotiates:

  • VIRTIO_BLK_F_RO — a read-only device is reported as DISK_STATUS_WR_PROTECT and write requests are rejected with -EROFS.

  • VIRTIO_BLK_F_FLUSH — DISK_IOCTL_CTRL_SYNC issues a flush to the device. When the device did not offer FLUSH there is no writeback cache to flush, so the ioctl is a successful no-op rather than an error.

A completed request whose status byte is not OK is logged and mapped to an errno: an unsupported request becomes -ENOTSUP and an I/O error -EIO.

VirtIO block configuration

DTS

A virtio,blk node is placed under its VirtIO transport parent and requires a disk-name used by the disk_access APIs. Over the PCI transport:

#include <zephyr/dt-bindings/pcie/pcie.h>

/ {
    pcie0 {
        virtio-blk-pci {
            compatible = "virtio,pci";
            vendor-id = <0x1af4>;
            device-id = <0x1001>;
            interrupts = <0xb 0x0 0x0>;
            interrupt-parent = <&intc>;
            status = "okay";

            virtio_blk: virtio-blk {
                compatible = "virtio,blk";
                disk-name = "VIRTIOBLK0";
                status = "okay";
            };
        };
    };
};

Over the MMIO transport the node is a child of an existing virtio,mmio bus:

&virtio_mmio4 {
    status = "okay";

    virtio_blk: virtio-blk {
        compatible = "virtio,blk";
        disk-name = "VIRTIOBLK0";
        status = "okay";
    };
};

Options

QEMU options

When CONFIG_DISK_DRIVER_VIRTIO_BLK is enabled on a QEMU board, the emulation layer creates a raw backing image and attaches it to the emulated virtio-blk device. PCI transports are attached from the common QEMU emulation code; the qemu_cortex_a53 board attaches an MMIO virtio-blk-device to virtio-mmio-bus.4.

Limitations

  • Exactly one request is in flight at a time. This matches the synchronous disk_access contract, so it costs that consumer no throughput, but the driver provides no request pipelining.

  • Like the rest of Zephyr’s VirtIO subsystem, the driver assumes coherent DMA and performs no cache maintenance on descriptors, rings, or data buffers.

  • Under an MMU the logical block size cannot exceed the MMU page size.