Re: [Qemu-devel] [PATCH] monitor: allow device to be ejected if no disk is inserted

[Kevin Wolf <kwolf@redhat.com>]

Am 02.06.2010 00:12, schrieb Eduardo Habkost:
> Resubmitting a patch that was submitted in December[1]. It was on the staging
> tree but somehow it got dropped. I have rebased it to current master branch on
> git.
> 
>   [1] http://article.gmane.org/gmane.comp.emulators.qemu/59813
> 
> --------
> 
> This changes the monitor eject_device() function to not check for
> bdrv_is_inserted().
> 
> Example run where the bug manifests itself:
> 
> (output of 'info block' is stripped to include only the CD-ROM device)
> 
>   (qemu) info block
>   ide1-cd0: type=cdrom removable=1 locked=0 [not inserted]
>   (qemu) change ide1-cd0 /dev/cdrom host_cdrom
>   (qemu) info block
>   ide1-cd0: type=cdrom removable=1 locked=0 file=/dev/cdrom ro=1 drv=host_cdrom encrypted=0
>   (qemu) eject ide1-cd0
>   (qemu) info block
>   ide1-cd0: type=cdrom removable=1 locked=0 file=/dev/cdrom ro=1 drv=host_cdrom encrypted=0
> 
>   # at this point, a disk was inserted on the host CD-ROM drive
> 
>   (qemu) info block
>   ide1-cd0: type=cdrom removable=1 locked=0 file=/dev/cdrom ro=1 drv=host_cdrom encrypted=0
>   (qemu) eject ide1-cd0
>   (qemu) info block
>   ide1-cd0: type=cdrom removable=1 locked=0 [not inserted]
>   (qemu)
> 
> The first eject command didn't work because the is_inserted() check
> failed.

But does it really make a difference? The guest should not see a medium
before and it should not see one afterwards.

> I have no clue why the code had the is_inserted() check, as it doesn't matter
> if there is a disk present at the host drive, when the user wants the virtual
> device to be disconnected from the host device.

The question is what the semantics of the eject monitor command is
supposed to be. I for one would have expected that it means that if
there was a medium inserted in the virtual CD-ROM drive, it won't be
there afterwards. I wouldn't have expected the connection to the host
device to be affected.

Actually, what I would have expected is not calling bdrv_close(), but
calling bdrv_eject() and possibly doing something with the device state
to reflect that. If the VM gets a real CD-ROM passed through, eject for
the virtual device should just mean eject for the real device.

> The is_inserted() check has another side effect: a memory leak if the "change"
> command is used multiple times, as do_change() calls eject_device() before
> re-opening the block device, but bdrv_close() is never called.

In the context of do_change the desired semantics is probably a
different one, I agree. It probably shouldn't call do_eject.

Kevin