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

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Message-ID: <20120213225623.GA2988@redhat.com>
Date:	Tue, 14 Feb 2012 00:59:06 +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 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?
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