Re: [PATCH v3] virtio-scsi: Send "REPORTED LUNS CHANGED" sense data upon disk hotplug events.

[Venu Busireddy <venu.busireddy@oracle.com>]

On 2022-09-30 18:25:48 +0200, Paolo Bonzini wrote:
> On Fri, Sep 30, 2022 at 4:42 PM Venu Busireddy
> <venu.busireddy@oracle.com> wrote:
> > > > Immediately after a hotunplug event, qemu (without any action from
> > > > the guest) processes a REPORT_LUNS command on the lun 0 of the device
> > > > (haven't figured out what causes this).
> > >
> > > There is only one call to virtio_scsi_handle_cmd_req_prepare and it
> > > takes the command from the guest, are you sure it is without any
> > > action from the guest?
> >
> > I am sure, based on what I am observing. I am running the scsitrace
> > (scsitrace -n vtioscsi -v) command on the Solaris guest, and I see no
> > output there.
> 
> Do you have the sources to the driver and/or to the scsitrace dtrace

I do not have access to the source code. I am working on gaining access.

> script? Something must be putting the SCSI command in the queue.
> Perhaps the driver is doing so when it sees an event? And if it is
> bypassing the normal submission mechanism, the REPORT LUNS commands is
> hidden in scsitrac; that in turn retruns a unit attention and steals

While SAM does say "if a REPORT LUNS command enters the enabled command
state, the device server shall process the REPORT LUNS command and shall
not report any unit attention condition;," it also says that the unit
attention condition will not be cleared if the UA_INTLCK_CTRL is set to
10b or 11b in the "Control mode page."

It doesn't appear to me that virtio-scsi supports "Control mode pages."
Does it? If it doesn't, is the expected handling of REPORT LUNS command
be same as the case of UA_INTLCK_CTRL being set to 00b?

And while trying to understand this, and reading the code regarding
the handling of UA_INTLCK_CTRL, I ran across the following comment in
scsi_req_get_sense():

/*
 * FIXME: clearing unit attention conditions upon autosense should be done
 * only if the UA_INTLCK_CTRL field in the Control mode page is set to 00b
 * (SAM-5, 5.14).
 *
 * We assume UA_INTLCK_CTRL to be 00b for HBAs that support autosense, and
 * 10b for HBAs that do not support it (do not call scsi_req_get_sense).
 * Here we handle unit attention clearing for UA_INTLCK_CTRL == 00b.
 */

If virtio-scsi doesn't support "Control mode pages," why does the above
comment even say "assume UA_INTLCK_CTRL to be 00b" or address the case
of 10b? Also, other than the reference to it in the above comment,
UA_INTLCK_CTRL is not used anywhere else in the code. This comment
confused me. Is the comment just wrong, or am I missing something? I am
just trying to understand this better so that I am better prepared when
the client driver folks start asking me questions about the qemu support.

Venu

> it from the other commands such as TEST UNIT READY, but that's a guest
> driver bug.
> 
> But QEMU cannot just return the unit attention twice. I would start
> with the patch to use the bus unit attention mechanism. It would be
> even better to have two unit tests that check the behavior prescribed
> by the standard: 1) UNIT ATTENTION from TEST UNIT READY immediately
> after a hotunplug notification; 2) no UNIT ATTENTION from REPORT LUNS
> and also no UNIT ATTENTION from a subsequent TEST UNIT READY command.
> Debugging the guest is a separate step.