Re: [Qemu-devel] [PATCH] Add HTTP protocol using curl v2

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, May 06, 2009 at 11:14:17AM +0200, Kevin Wolf wrote:
> Daniel P. Berrange schrieb:
> > On Wed, May 06, 2009 at 03:00:50AM +0200, alex@csgraf.de wrote:
> >> From: Alexander Graf <alex@csgraf.de>
> >>
> >> Currently Qemu can read from posix I/O and NBD. This patch adds a
> >> third protocol to the game: HTTP.
> >>
> >> In certain situations it can be useful to access HTTP data directly,
> >> for example if you want to try out an http provided OS image, but
> >> don't know if you want to download it yet.
> >>
> >> Using this patch you can now try it on on the fly. Just use it like:
> >>
> >> qemu -cdrom http://host/path/my.iso
> > 
> > I rather think there should be an explicit flag to allow use of http://
> > URLs in filenames at runtime, not just 'configure' time. There are many
> > apps out there using QEMU which will be assuming QEMU treats all disk
> > paths as local files, and thus not got explicit code to check whether 
> > a URI is passed. I could well see that some will consider it a security
> > issue to allow QEMU to download off the net, but if they updated to
> > a new QEMU with this patch, downloading would be allowed by default.
> 
> If apps want to be sure that they are accessing a local file, they must
> ensure not to have a colon in the file name. Otherwise this specifies a
> protocol for the qemu block layer.

Using paths with a colon in the file name is pretty critical if
you wish to configure VMs with block devices using a stable path
like those under /dev/disk/by-path/ - they pretty much all contain
colons, and are the only good stable path for iSCSI or FibreChannel
block devices.

> Btw, we could use a way to escape colons in a file name. Using such
> files isn't possible currently.
> 
> > Perhaps only enable these remote URIs with the -drive parameter, when
> > an explicit fmt=http option is set. But can this be layered into the
> > other protocols, eg could the remote URI be in qcow, vmdk, etc formats,
> > or are you assuming the remote uri is raw file ?
> 
> It should work with all formats. This is why fmt=http is wrong. It's not
> a format, but a protocol.

Then I'd prefer we add a protocol=XXX option for magic protocols. This 
would be easier to use & clearer than requiring escaping of magic 
characters, eg

   -drive file=/some/path:with:colons,protocol=file
   -drive file=http://some/path,protocol=uri

For compatability, we could make it such that if protocol=XXX was left out,
it could try and "guess" it, in same way QEMU does if format=XXX is left out
for content format.

Daniel
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