[Qemu-devel] Re: [PATCH 0/2] Guess host device type from guest device type for block devices

[Anthony Liguori <anthony@codemonkey.ws>]

Eduardo Habkost wrote:
> This fixes the following issue:
> https://bugzilla.redhat.com/show_bug.cgi?id=473154
>
> Sometimes a CD-ROM drive path doesn't start with /dev/cdrom, causing problems
> if a disk isn't inserted on the drive. With this patch, the floppy and cdrom
> drivers are used if using a host block device as backend and the
> virtual device type is floppy or CD-ROM.
>   

Filename based probing is bad, but I think your solution is flawed too.  
It breaks the case where someone does:

qemu -hda /dev/volumes/RH5-iso

Where /dev/volumes/RH5-iso is just a plain LVM volume.  I can think of 
legitimate reasons to do this.

I think a better approach would be to try and issue a cdrom-specific 
ioctl without side-effects, and then use that probe to decide whether 
it's a cdrom.  For instance, CDROM_DRIVE_STATUS ought to be pretty safe.

Long term, I think we want to take a more thorough approach to host 
devices.  First, we should use fstat instead of filenames to determine 
whether something is a host device.

We should then use the info in fstat to determine what type of device it 
is.  Then we can use host-specific mechanisms to determine what the host 
device type is.

For instance, on Linux, the right thing to do is:

lstat(path) -> test -e /sys/dev/block/<major>:<minor>/capability -> 
capability & GENHD_FL_CD

Regards,

Anthony Liguori