Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

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On Tue, Feb 14, 2012 at 7:42 AM, ronnie sahlberg
<ronniesahlberg@xxxxxxxxx> wrote:
> On Tue, Feb 14, 2012 at 2:12 AM, Hannes Reinecke <hare@xxxxxxx> wrote:
>> On 02/13/2012 02:18 PM, Michael S. Tsirkin wrote:
>>> On Tue, Feb 14, 2012 at 12:13:36AM +1100, ronnie sahlberg wrote:
>>>> On Tue, Feb 14, 2012 at 12:00 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>>>>> On Mon, Feb 13, 2012 at 02:54:03PM +0200, Dor Laor wrote:
>>>>>> Only if you use the pci multi-function option but that kills
>>>>>> standard hot unplug
>>>>>
>>>>> It doesn't kill it as such, rather you can't unplug luns individually.
>>>>
>>>> Isnt that just a consequence of the current implementation rather than
>>>> a SCSI limitation?
>>>
>>> Yes.
>>>
>>>> A different way to do hoplug could be to flag all devices as removable
>>>> in the standard inq page then
>>>> leave the LUN there persistently and what you remove/add is not the
>>>> LUN device itself but just the media in the device.
>>>>
>>>> Instead of hot-plug remove the LUN,  hot-plug becomes "media eject" or
>>>> "media insert".
>>>> The device remains present all time, you never remove it, but instead
>>>> hot-plug controls if the media is present or not.
>>>>
>>>>
>>>> This would require implementing at least START_STOP_UNIT and
>>>> PREVENT_ALLOW_MEDIUM_REMOVAL opcode emulation from SBC.
>>>>
>>>>
>>>> regards
>>>> ronnie sahlberg
>>>
>>> That would work.
>>>
>> Or we simply use the Peripheral Qualifier that the device is gone;
>> eg we could simply set PQ = 1, return sense code 0x25/00 and be done
>> with ...
>>
>
> That is still similar to "rip a device out from the guest without notice"
> and can cause the guest to be "surprised".
>
>
> Removable media is standard feature in SCSI SBC (and other commandsets).
> The nice part of removable media is that it activates a contract
> between the device and the guest
> to prevent removal of the media when the guest depends on the media
> not being removed.
>
> I.e.  If you have a SBC device with the removable-media bit set,
> this is used to tell the initiator "this media can be removed, be
> prepared that this might happen".
> So when you mount such a SBC device in the guest, the guest will issue
> a "PREVENT_ALLOW_MEDIUM_REMOVAL"
> to tell the device "this medium is in use and may not be removed".
>

What I mean is that if /dev/sdb is removable,
if you mount this as   "mount /dev/sdb1 /mnt"
this will automatically cause the guest kernel to send a
PREVENT_ALLOW_MEDIUM_REMOVAL to /dev/sdb to prevent removal.

When you "umount /dev/sdb1"   the kernel/guest will automagically send
PREVENT_ALLOW_MEDIUM_REMOVEAL to /dev/sdb and allow removal of the
media again.


If you capture this command and track the "prevent/allow removal
status"  you automatically get a channel where qemu will
know when it is safe to unplug the device  and when it is not safe to
unplug the device.
This is a nice feature.
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