Re: [Qemu-devel] Mount image file feature

[Max Reitz <mreitz@redhat.com>]

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On 31.08.2015 22:13, Programmingkid wrote:
> 
> On Aug 29, 2015, at 12:39 PM, Max Reitz wrote:
> 
>> On 29.08.2015 17:57, Programmingkid wrote:
>>>
>>> On Aug 29, 2015, at 11:40 AM, Max Reitz wrote:
>>>
>>>> On 27.08.2015 03:05, G 3 wrote:
>>>>> I want to share files between my host and guest computer. A feature I
>>>>> want to add would be a new menu item in the Machine menu called "Mount
>>>>> Image File...". When the user selects it, a file open dialog box
>>>>> displays. The user can then select the image file with the file he wants
>>>>> to use. After pushing the OK button, the image file would be mounted
>>>>> like a USB flash drive. This menu item would only show up if there is
>>>>> usb support in the guest machine.
>>>>>
>>>>> Would you be open to accepting such a feature?
>>>>
>>>> Generally I'd expect this to be functionality exposed by the management
>>>> layer. For instance using virt-manager, this can be achived as follows:
>>>> Switch to "Details", then click "Add Hardware", choose "Storage" and
>>>> "USB" as the "Bus type". Choose the image, click "Finish", done.
>>>
>>> Isn't Libvirt only available on Linux? This mount image file feature would
>>> only be on Mac OS X.
>>
>> I'm not sure whether that sounds like a good idea, because then people
>> using bare qemu on Linux would complain that it isn't available with
>> Gtk. So if this was to be implemented, it would have to implemented
>> cross-platform (or at least in a way so it can be used cross-platform
>> later on).
>>
>>> Mac OS X users don't have all the fancy GUI wrappers
>>> for QEMU :(
>>
>> Good thing most GNU/Linux distributions are free. ;-)
>>
>> (sorry, could not resist)
>>
>>> Mac OS X is a second-class citizen in the QEMU world...
>>
>> Might have to do something with most (?) of it being non-free and Apple
>> not caring enough about KVM.
>>
>> (And without KVM, people in turn don't care enough about OS X as a qemu
>> host.)
>>
>> ((But all of that is pretty biased speculation, of course.))
>>
>>>> The main problem I see with adding this functionality to qemu itself
>>>> would be having to get even further into the GUI business, which hasn't
>>>> worked out too well so far…
>>>
>>> That is because of several reasons. One being maintainers not wanting to
>>> advance the GUI because they feel another program should be QEMU's 
>>> GUI. I'm sure there are plenty of good ideas that would advance QEMU's
>>> GUI. These ideas just need to be accepted into QEMU rather than put off.
>>
>> Another is that some people simply feel that qemu should focus on being
>> a backend than having to mess with frontend work, too. See the recent
>> discussion on the Gtk code setting the locale and thus breaking QMP for
>> an example why they have a point.
>>
>> I guess you'll better talk to Markus about this. :-)
>>
>> Quote: "We should've stayed out of the GUI business."
>>
>> (http://lists.nongnu.org/archive/html/qemu-devel/2015-08/msg03049.html)
>>
>>>> If we didn't care about that, than we'd have to think about the
>>>> implementation. Internally, we'd probably call QMP's blockdev-add to
>>>> open the image file, and then QMP's device_add to add the USB device. So
>>>> then qemu would use its own management interfaces to execute the
>>>> operation, which seems a bit strange to me, further hinting at the fact
>>>> that we probably should leave this to the management layer.
>>>
>>> What works does, and it isn't always as nice looking
>>> as we want it. I am sure we will use some kind of API to implement this feature.
>>
>> Having to deal with ugly legacy cruft from time to time, I don't know
>> whether "What works, works" is always appropriate.
>>
>>> I just wish there were an easy way to share files between the host and the guest.
>>
>> I don't think using emulated USB storage is the right way to do this,
>> though. Stefan is working on file sharing using NFS over virtio-vsock,
>> which seems more appropriate. But then again I don't whether
>> virtio-vsock will work with an OS X host…
>>
>> ===
>>
>> OK, if you really want to implement it, I'm certainly not the right one
>> to stop you, so here is how I'd do it:
>>
>> My "BlockBackend and media" series rewrites the "change" HMP/QMP command
>> to be a macro, basically, that actually executes four lower-level QMP
>> commands. So this means we have a precedent of "macro" QMP commands, and
>> this could be extended. So you could add a "macro" QMP command
>> "usb-storage-insert-file" or something which executes blockdev-add +
>> device_add (if that works).*
>>
>> Then, if I felt really fancy, I'd add some layer which allows
>> generically executing QMP commands through the GUI, based on a whitelist
>> of commands. Each parameter would have to be requested through some GUI
>> interface, for instance, filenames would be queried through an
>> appropriate dialog. Ideally, this would be GUI-agnostic, but this may
>> not be reasonably possible.
>>
>> Then you'd whitelist usb-storage-insert-file (or however it is named),
>> give it some nice alias and you'd be done.
>>
>> While this would be much work I feel like this would actually be the
>> nicest solution.
>>
>> This is just a very rough outline, though, and since it somehow goes
>> against everything qemu's GUI was used for so far (just the most basic
>> things, basically nothing about controlling the VM except for
>> Pause/Shutdown/Reboot) I have no idea how it would be received.
>>
>> Max
>>
>>
>> *Actually you'd probably want a generic insert-storage-file which takes
>> the kind of storage device to add as a parameter.
> 
> I thought about using add_init_drive() found in device-hotplug.c, 
> but it is private. Too bad. It looked perfect.  
> 
> https://wiki.ubuntu.com/QemuDiskHotplug#Hotplug_USB_Disk
> This page say talks about how to do it. This is what it said to do:
> 
> drive_add 0 if=none,id=usbdisk1,file=/tmp/test.img
> 
> Then
> 
> device_add usb-storage,id=usbdisk1,drive=usbdisk1
> 
> I wasn't able to follow what you said. Do you think you could send me
> an example of how you think I should do the mounting of the image
> file?

That was the "if that works" part. ;-)

The following works for me:

$ echo foo > bar
$ x86_64-softmmu/qemu-system-x86_64 -qmp stdio -usb -cdrom
~/tmp/archlinux-2015.07.01-dual.iso -enable-kvm -m 512
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 4, "major": 2},
"package": ""}, "capabilities": []}}
{'execute': 'qmp_capabilities'}
{"return": {}}
{'execute': 'blockdev-add', 'arguments': {'options': {'id': 'usb-image',
'driver': 'raw', 'file': {'driver': 'file', 'filename': 'bar'}}}}

{"return": {}}
{'execute': 'device_add', 'arguments': {'driver': 'usb-storage', 'id':
'usb-disk', 'drive': 'usb-image'}}
{"return": {}}

In the VM, before device_add:
# cat /dev/sda
cat: /dev/sda: No such file or directory

After device_add:
# cat /dev/sda
foo

Unplugging the device can be done with device_del; but there is no
blockdev-del yet, so the image file will remain lingering.

Max


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