Re: [PATCH v3 4/4] block: switch to use qemu_open/qemu_create for improved errors

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> Currently at startup if using cache=none on a filesystem lacking
> O_DIRECT such as tmpfs, at startup QEMU prints
>
> qemu-system-x86_64: -drive file=/tmp/foo.img,cache=none: file system may not support O_DIRECT
> qemu-system-x86_64: -drive file=/tmp/foo.img,cache=none: Could not open '/tmp/foo.img': Invalid argument
>
> while at QMP level the hint is missing, so QEMU reports just
>
>   "error": {
>       "class": "GenericError",
>       "desc": "Could not open '/tmp/foo.img': Invalid argument"
>   }
>
> which is close to useless for the end user trying to figure out what
> they did wrong.
>
> With this change at startup QEMU prints
>
> qemu-system-x86_64: -drive file=/tmp/foo.img,cache=none: Unable to open '/tmp/foo.img' flags 0x4000: filesystem does not support O_DIRECT
>
> while at the QMP level QEMU reports a massively more informative
>
>   "error": {
>      "class": "GenericError",
>      "desc": "Unable to open '/tmp/foo.img' flags 0x4002: filesystem does not support O_DIRECT"
>   }
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

Subject promises slightly more than the patch actually provides:

    block/vvfat.c:1355:        int fd = qemu_open(mapping->path, O_RDONLY | O_BINARY | O_LARGEFILE);
    block/vvfat.c:2516:    fd = qemu_open(mapping->path, O_RDWR | O_CREAT | O_BINARY, 0666);

If you'd rather not touch block/vvfat.c (I understand), consider
tweaking the subject to say

    block/file: ...