[Qemu-devel] Re: [PATCH v2 1/2] cdrom: Allow the TEST_UNIT_READY command after a cdrom change

[Kevin Wolf <kwolf@redhat.com>]

Am 07.04.2011 11:11, schrieb Amit Shah:
> On (Thu) 07 Apr 2011 [10:59:20], Kevin Wolf wrote:
>> Am 07.04.2011 07:05, schrieb Amit Shah:
>>> We restrict the commands that a guest can send us after a cdrom change
>>> event.  The current list includes REQUEST_SENSE and INQUIRY commands.
>>> Guests can also issue TEST_UNIT_READY to inquire for the status, so
>>> allow this command as well.
>>>
>>> Signed-off-by: Amit Shah <amit.shah@redhat.com>
>>
>> Hm... MMC-5, section 4.1.6.1 seems to conflict with this:
>>
>> "If a Host issues a command other than GET CONFIGURATION, GET EVENT
>> STATUS NOTIFICATION, INQUIRY or REQUEST SENSE while a unit attention
>> condition exists for that Host, the Drive shall not perform the command
>> and shall report CHECK CONDITION status unless a higher priority status
>> as defined by the Drive is also pending."
>>
>> So while you're right that our list is incomplete, TEST UNIT READY
>> doesn't seem to be among the missing commands.
> 
> Hm - older Linux guests (pre 2.6.38) and Windows guests, as Gleb's
> commit mentioned, rely on this command to get CD change notifications:
> 
> /* identical to scsi_test_unit_ready except that it doesn't
>  * eat the NOT_READY returns for removable media */
> int sr_test_unit_ready(struct scsi_device *sdev, struct scsi_sense_hdr *sshdr)
> {
>         int retries = MAX_RETRIES;
>         int the_result;
>         u8 cmd[] = {TEST_UNIT_READY, 0, 0, 0, 0, 0 };
> 
>         /* issue TEST_UNIT_READY until the initial startup UNIT_ATTENTION
>          * conditions are gone, or a timeout happens
>          */
>         do {
>                 the_result = scsi_execute_req(sdev, cmd, DMA_NONE, NULL,
>                                               0, sshdr, SR_TIMEOUT,
>                                               retries--, NULL);
>                 if (scsi_sense_valid(sshdr) &&
>                     sshdr->sense_key == UNIT_ATTENTION)
>                         sdev->changed = 1;
> 
>         } while (retries > 0 &&
>                  (!scsi_status_is_good(the_result) ||
>                   (scsi_sense_valid(sshdr) &&
>                    sshdr->sense_key == UNIT_ATTENTION)));
>         return the_result;
> }

I think the scsi_execute_req() call might issue a REQUEST SENSE
internally and therefore clear the unit attention condition. Tried to
check that in the source, but I'm hopelessly lost in the kernel...

Kevin