Starting QEMU with `-drive aio=io_uring` fails with `invalid aio option`

[volkertb]

In Clear Linux version 34320, when you enter man qemu and scroll down to the available options under -drive, you find the following information:

aio=aio
       aio  is  "threads",  "native", or "io_uring" and selects between pthread based disk I/O, native Linux AIO, or Linux
       io_uring API.

All these three options are also listed when you run qemu with the --help option:

$ qemu-system-x86_64 --help | grep aio
       [,aio=threads|native|io_uring]

I've heard good things about the io_uring feature, so naturally, I'd like to try it. However, when I try to start QEMU with a drive configured with the aio mode set to io_uring, I get the following result:

$ qemu-system-x86_64 -drive aio=io_uring,if=virtio,format=qcow2,file=/some/file.qcow2
qemu-system-x86_64: -drive aio=io_uring,if=virtio,format=qcow2,file=/some/file.qcow2: invalid aio option

The invalid aio option error is shown whenever you enter a non-existent aio option, like this:

$ qemu-system-x86_64 -drive aio=blabla,if=virtio,format=qcow2,file=/some/file.qcow2
qemu-system-x86_64: -drive aio=blabla,if=virtio,format=qcow2,file=/some/file.qcow2: invalid aio option

When I specify threads or native, it works fine.

I've tried the same on a Fedora 33 system and on an Ubuntu 20.10 system, and in both cases, aio=io_uring was recognized without this error. So apparently, it's specific to Clear Linux.

Even if there's a technical limitation currently preventing aio=io_uring from working with QEMU in Clear Linux, I would expect a more specific and explanatory error to be shown, not the same one that you would get when you enter any invalid value. For instance, a GitHub search yielded this result in the QEMU source code:

#ifdef CONFIG_LINUX_IO_URING
    if (s->use_linux_io_uring) {
        if (!aio_setup_linux_io_uring(bdrv_get_aio_context(bs), errp)) {
            error_prepend(errp, "Unable to use io_uring: ");
            goto fail;
        }
    }

Direct link to this code: https://github.com/qemu/qemu/blob/1ed9228f63ea4bcc0ae240365305ee264e9189ce/block/file-posix.c#L696

Please let me know if I can help in testing or troubleshooting this further. Thanks.

[mrartanis]

Face same issue with qemu builded on machine without liburing-dev (qemu-system-x86_64 --help | grep aio returned [,aio=threads|native|io_uring] too) Rebuild package with liburing and aio=io_uring start working.

[fenrus75]

adding iouring now.. should be in the next release

On Tue, Jan 18, 2022 at 6:24 AM Artanis @.***> wrote:

Face same issue with qemu builded on machine without liburing-dev (qemu-system-x86_64 --help | grep aio returned [,aio=threads|native|io_uring] too) Rebuild package with liburing and aio=io_uring start working.

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