linux-kernel - Re: [PATCH] sd: do not set changed flag on all unit attention conditions

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Date:	Tue, 17 Jul 2012 10:34:24 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH] sd: do not set changed flag on all unit attention conditions

Il 17/07/2012 09:45, James Bottomley ha scritto:
> On Mon, 2012-07-16 at 19:20 +0200, Paolo Bonzini wrote:
>> Il 16/07/2012 18:18, James Bottomley ha scritto:
>>>>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>>>>> index b583277..6d8ca08 100644
>>>>> --- a/drivers/scsi/scsi_lib.c
>>>>> +++ b/drivers/scsi/scsi_lib.c
>>>>> @@ -843,8 +843,11 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
>>>>>  	} else if (sense_valid && !sense_deferred) {
>>>>>  		switch (sshdr.sense_key) {
>>>>>  		case UNIT_ATTENTION:
>>>>> -			if (cmd->device->removable) {
>>>>> -				/* Detected disc change.  Set a bit
>>>>> +			if (cmd->device->removable &&
>>>>> +			    (sshdr.asc == 0x3a ||
>>>>> +			     (sshdr.asc == 0x28 && sshdr.ascq == 0x00))) {
>>>>> +				/* "No medium" or "Medium may have changed."
>>>>> +				 * This means a disc change.  Set a bit
>>> This type of change would likely cause a huge cascade of errors in real
>>> removable media devices.  Under the MMC standards, which a lot of the
>>> older removable discs seem to follow, UNIT ATTENTION indicates either
>>> medium change or device reset (which we check for and eat lower down);
>>> we can't rely on them giving proper SBC-2 sense codes.  If you want to
>>> pretend to be removable media, you have to conform to its standards.
>>
>> Would you accept a patch doing the opposite, i.e. passing some sense
>> codes such as PARAMETERS CHANGED and TARGET OPERATING CONDITIONS HAVE
>> CHANGED?
> 
> Could you explain what the problem actually is?  It looks like you had a
> reason to mark virtio-scsi as removable, even though it isn't, and now
> you want to add further hacks because being removable doesn't quite
> work.

It's not specific to virtio-scsi, in fact I expect that virtio-scsi will
be almost always used with non-removable disks.

However, QEMU's SCSI target is not used just for virtio-scsi (for
example it can be used for USB storage), and it lets you mark a disk as
removable---why? because there exists real hardware that presents itself
as an SBC removable disk.  The only thing that is specific to
virtualization, is support for online resizing (which generates a unit
attention condition CAPACITY DATA HAS CHANGED).

Paolo
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