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

From: Paolo Bonzini
Date: Tue Jul 17 2012 - 04:34:31 EST

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
> 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).

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