-device does not work as -drive do

[Hibou57 (Yannick Duchêne)]

Copy/paste of https://stackoverflow.com/questions/52929723/qemu-eject-complains-device-is-not-found-while-it-is-there , since I found this bug trying to find an answer to an own question on Stack Overflow.

Below, what was my question the answer I wrote, all exposes the bug.

====================================================================

I need to eject a floppy from QEmu 3.0 monitor, but the command surprisingly fails complaining the device is not found, while it is really there.

Listing of devices:

    (qemu) info block
    fda: dos-6-22/Dos622-1.img (raw)
        Attached to: /machine/unattached/device[11]
        Removable device: not locked, tray closed
        Cache mode: writeback

    hda: hda.img (raw)
        Attached to: /machine/peripheral-anon/device[1]
        Cache mode: writeback

Eject command result:

    (qemu) eject fda
    Device 'fda' not found

This is so although this documentation says this is how I have to do: https://www.linux-kvm.org/page/Change_cdrom (just that I want to eject the floppy instead of the CD‑ROM).

The `change` command complains the same:

    (qemu) change fda dos-6-22/Dos622-2.img raw
    Device 'fda' not found

Is this a bug or me doing something wrong?

I tried using different node names, with always the same result.

====================================================================

I’m posting as an answer, but I’m not strictly sure. I can just say, if I understand correctly, this is a bug.

The answer comes in two parts.

First part, is a stripped down failing invocation:

    qemu-system-i386 \
       -monitor stdio \
       -machine type=isapc,vmport=off \
       -blockdev driver=file,node-name=fda-img,filename=fda.img \
       -blockdev driver=raw,node-name=fda,file=fda-img \
       -global isa-fdc.driveA=fda

    (qemu) info block
    ide1-cd0: [not inserted]
        Attached to: /machine/unattached/device[19]
        Removable device: not locked, tray closed

    sd0: [not inserted]
        Removable device: not locked, tray closed

    fda: fda.img (raw)
        Attached to: /machine/unattached/device[13]
        Removable device: not locked, tray closed
        Cache mode: writeback
    (qemu) eject fda
    Device 'fda' not found

Second part, is the same without the last argument `-global isa-fdc.driveA=fda`:

    qemu-system-i386 \
       -monitor stdio \
       -machine type=isapc,vmport=off \
       -blockdev driver=file,node-name=fda-img,filename=fda.img \
       -blockdev driver=raw,node-name=fda,file=fda-img

    (qemu) info block
    ide1-cd0: [not inserted]
        Attached to: /machine/unattached/device[19]
        Removable device: not locked, tray closed

    floppy0: [not inserted]
        Attached to: /machine/unattached/device[13]
        Removable device: not locked, tray closed

    sd0: [not inserted]
        Removable device: not locked, tray closed
    (qemu) eject floppy0

There is more error when `-global isa-fdc.driveA=fda` is removed. However, the documentation says:

> -global driver=driver,property=property,value=value
> Set default value of driver’s property prop to value, e.g.:

> qemu-system-i386 -global ide-hd.physical_block_size=4096 disk-image.img
> In particular, you can **use this to set driver properties for devices which are created automatically by the machine model**. To create a device which is not created automatically and set properties on it, use -device.

> -global driver.prop=value is shorthand for -global driver=driver,property=prop,value=value. The longhand syntax works even when driver contains a dot.

What I put a stress on in the quote, suggest I’m not misusing `-global` and that’s most probably a bug.

**Update for more details:**

It seems using `-drive` instead of `-device` and `driveA` assignment, the result is not the same, although RedHat documentation recommands using `-device` instead of `-drive` and QEmu 3.0 documentation says `-drive` is essentially a shortcut for `-device` (“essentially”, not telling about the difference).

Below, two cases, with an except of `info block` and an excerpt of `info qtree`.

With this one, `eject floppy0` works:

    qemu-system-i386 \
       -monitor stdio \
       -machine type=isapc,vmport=off \
       -drive format=raw,if=floppy,media=disk,file=fda.img \
       -device isa-vga,vgamem_mb=1 \
       -serial msmouse

    […]

    floppy0 (#block156): fda.img (raw)
        Attached to: /machine/unattached/device[12]
        Removable device: not locked, tray closed
        Cache mode: writeback

    […]

      dev: isa-fdc, id ""
        iobase = 1008 (0x3f0)
        irq = 6 (0x6)
        dma = 2 (0x2)
        driveA = ""
        driveB = ""
        check_media_rate = true
        fdtypeA = "auto"
        fdtypeB = "auto"
        fallback = "288"
        isa irq 6
        bus: floppy-bus.0
          type floppy-bus
          dev: floppy, id ""
            unit = 0 (0x0)
            drive = "floppy0"
            logical_block_size = 512 (0x200)
            physical_block_size = 512 (0x200)
            min_io_size = 0 (0x0)
            opt_io_size = 0 (0x0)
            discard_granularity = 4294967295 (0xffffffff)
            write-cache = "auto"
            share-rw = false
            drive-type = "144"

With this one, `eject fda` does not work:

    qemu-system-i386 \
       -monitor stdio \
       -machine type=isapc,vmport=off \
       -blockdev driver=file,node-name=fda-img,filename=fda.img \
       -blockdev driver=raw,node-name=fda,file=fda-img \
       -global isa-fdc.driveA=fda \
       -device isa-vga,vgamem_mb=1 \
       -serial msmouse

    […]

    fda: fda.img (raw)
        Attached to: /machine/unattached/device[12]
        Removable device: not locked, tray closed
        Cache mode: writeback

    […]

      dev: isa-fdc, id ""
        iobase = 1008 (0x3f0)
        irq = 6 (0x6)
        dma = 2 (0x2)
        driveA = ""
        driveB = ""
        check_media_rate = true
        fdtypeA = "auto"
        fdtypeB = "auto"
        fallback = "288"
        isa irq 6
        bus: floppy-bus.0
          type floppy-bus
          dev: floppy, id ""
            unit = 0 (0x0)
            drive = "fda"
            logical_block_size = 512 (0x200)
            physical_block_size = 512 (0x200)
            min_io_size = 0 (0x0)
            opt_io_size = 0 (0x0)
            discard_granularity = 4294967295 (0xffffffff)
            write-cache = "auto"
            share-rw = false
            drive-type = "144"

[Max Reitz (xanclic)]

Hi,

As you yourself say, -drive does work, so this is really not about -drive, but about -blockdev.

-drive creates part of the device as well, so to speak, whereas -blockdev only creates one (or more) nodes in the block layer. You can only eject something from devices, however, so you cannot eject a blockdev. So yes, there is a difference between -drive and -blockdev.

If you want to use eject, you either have to use -drive (which is legacy), or you eject the ID given to -device (because you eject the device). However, you haven't even used -device, but you just used -global (which doesn't allow giving an ID).

As you can see from "info block", there is still a QOM path that you could in theory use, too. So you probably could also "eject /machine/unattached/device[12]" (in the last example).

In practice, you should probably do this:

$ qemu-system-i386 \
       -blockdev driver=file,node-name=fda-img,filename=fda.img \
       -blockdev driver=raw,node-name=fda,file=fda-img \
       -device floppy,id=floppy,drive=fda \
       -monitor stdio
(qemu) info block
[...]
fda: fda.img (raw)
    Attached to: floppy
    Removable device: not locked, tray closed
    Cache mode: writeback
(qemu) eject floppy
Device 'floppy' not found

Oops.

Well, it works with QMP:

{'execute':'eject','arguments':{'id':'floppy'}}
{"return": {}}

So there is indeed a bug, although it's not the one you're reporting. The bug is that HMP's eject does not accept a -device ID (but only a -drive ID).

Max

[Hibou57 (Yannick Duchêne) (yannick-duchene)]

Yes, I messed up with `-device`, because I initially tried to do it with `-device`, later rely only on automatically created device, and so had automatically created devices.

Thanks for you rich comment, I will study it.

[Thomas Huth (th-huth)]

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