proper config for qemu's host_cdrom

[daggs]

Greetings Peter,

> Sent: Monday, November 02, 2020 at 4:20 PM
> From: "Peter Krempa" <pkrempa at redhat.com>
> To: "daggs" <daggs at gmx.com>
> Cc: libvirt-users at redhat.com
> Subject: Re: proper config for qemu's host_cdrom
>
> Note that all of the above is not "SCSI passthrough" but rather device
> emulation backed with a real cdrom. This is fine if you want to use the
> cdrom for block operations (reading) CDs.
>
> In that case qemu must use "host_cdrom" to provide functionality such as
> eject.
>
> For true SCSI passtrhrough, when all commands are passed to the host
> device (good also for other operations, e.g. writing CDs)
>
> For such operation the XML using <hostdev> ...
>
> ... as you've shown here is used.
>
> In such case we use 'host_device' even for cdroms. qemu only allows to
> use the 'host_device' protocol backend.
>
> The main difference is that for the emulation interface (<disk>) we use
> the block interface for the cdrom (/dev/sr0 for example). For hostdev we
> open the SCSI-generic interface which is a character device and is used
> to pass generic commads (/dev/sg0)
>
> I've originally attempted to change the way we open /dev/sg0 as you
> might know since you were CC'd on that thread, but testing actually
> proved that the above approach is correct, for scsi passthrough we don't
> need to treat the cdrom differently.
>
>

so host_device is the right way in this case? if so, any issues I see in the vm should be sent to qemu ml right?

Dagg,