Re: [Qemu-devel] Mount image file feature

[Programmingkid <programmingkidx@gmail.com>]

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?