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

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Date:	Tue, 14 Feb 2012 01:33:35 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	ronnie sahlberg <ronniesahlberg@...il.com>
Cc:	Hannes Reinecke <hare@...e.de>, Dor Laor <dlaor@...hat.com>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christian Hoff <christian.hoff@...ibm.com>,
	borntrae@...ux.vnet.ibm.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	rusty@...tcorp.com.au, Stefan Hajnoczi <stefanha@...il.com>,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

On Tue, Feb 14, 2012 at 10:30:59AM +1100, ronnie sahlberg wrote:
> On Tue, Feb 14, 2012 at 9:59 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > On Tue, Feb 14, 2012 at 07:53:26AM +1100, ronnie sahlberg wrote:
> >> On Tue, Feb 14, 2012 at 7:42 AM, ronnie sahlberg
> >> <ronniesahlberg@...il.com> wrote:
> >> > On Tue, Feb 14, 2012 at 2:12 AM, Hannes Reinecke <hare@...e.de> 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@...hat.com> 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.
> >
> > Presumably there's a way for device to notify the OS
> > that user requested removal, as well?
> 
> 
> I think that is done by responding with sense  to one of the commands,
> like the every few second TEST_UNIT_READY that the
> initiator/guest-kernel will send.

Does it do this even for mounted media?
I didn't realize ...
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