Re: [Qemu-devel] To O_EXCL or not to O_EXCL open host_cdrom

[Avi Kivity]

On 04/08/2011 02:33 PM, Stefan Hajnoczi wrote:
> Amit and I were discussing the pros and cons of using O_EXCL to open
> host CD-ROM devices on IRC but this discussion could benefit from more
> input.
>
> Linux block devices (like /dev/sr0 CD-ROMs) can be opened with O_EXCL
> and only one userspace process will succeed at a time.  This prevents
> programs from interfering with each other.  The polling daemons, hald
> and udisks, use O_EXCL and mount does too.
>
> Today QEMU does not use O_EXCL and will therefore access host CD-ROMs
> while they are in use by other programs.  This also means that
> programs can be started on the host while QEMU is already running that
> may interfere with the virtual machine's ability to access the CD-ROM
> (for example by ejecting it).

Also, performance would likely be ruined if the disk wasn't cached 
(likely with a DVD).  CD-ROMs seek very slowly.  It would be even 
funnier if we supported cd-writers.

> Therefore, it sounds reasonable to switch to O_EXCL to prevent
> interfering with other programs and to prevent other programs
> interfering with QEMU.
>
> On the downside, it will no longer be possible to share a host CD-ROM
> between multiple virtual machines or to mount it on host while passing
> it through to a guest.  These scenarios are not safe because on of the
> clients could eject the device, spoiling the party for everyone else.
> However, it is a handy feature for putting installation media into a
> machine and installing several guests at the same time.

You'd probably benefit a lot more by copying the media to disk.

> The other concern I have about using O_EXCL is that we expose
> ourselves to race conditions if there is ever a need to re-open the
> device.  When QEMU closes its file descriptor another program may be
> scheduled to run and open the device with O_EXCL.  Now QEMU will not
> be able to open the CD-ROM anymore.  From the guest perspective this
> could be at an odd time and we'd have to start failing requests.
> Today we do not re-open host CD-ROMs though so this isn't a pressing
> problem.

We really should avoid re-open whenever possible.

The standard response to such questions is "let's add another option", 
but in this case I don't think we should.  Like Amit says, this is a 
niche use case.

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