Re: [Qemu-devel] [PATCH] Add readonly flag to -drive command

[Kevin Wolf]

Am 12.10.2009 18:15, schrieb Michael Tokarev:
> Anthony Liguori wrote:
>> Jamie Lokier wrote:
>>> Kevin Wolf wrote:
>>>  
>>>> Am 12.10.2009 14:47, schrieb Naphtali Sprei:
>>>>    
>>>>> In order to safely share an image between guests (as read only 
>>>>> drive), add a 'readonly' flag
>>>>> to the -drive command (qemu command line and monitor).
>>>
>>> Heh.  I've been sharing images between guests for ages - using "chmod -r" 
>>> :-)
>>
>> Were it not for backwards compatibility, I would remove that and have it 
>> error out.
>>
>> I've had multiple people end up with strangely broken guests because 
>> they didn't realize the image was read only.
> 
> And I had this issue too.
> 
> But for me it's still not clear.  Some disk drives out there can be
> switched to read-only mode at runtime by using appropriate command.
> Certain (USB and especially SD) flash drives can be read-only too
> (usually a tiny swith does that).  Even old good floppy drives had
> a "write protect" switch.  How the system determines if such devices
> are read-only?  For a HDD or a flash drive, I *guess* the drive return
> appropriate error message, no?
> 
> But qemu - apparently - just ignores writes in this case, which
> results in stalled guest which expects some answer to the write
> command..  I guess, because it was always the stalls.

The qemu block layer returns errors. For good old bdrv_write it's
-EACCES, for bdrv_aio_writev it's NULL (and therefore cannot be
distinguished from other errors). Not sure what the devices make out of
it. Probably not the right thing in all cases.

Kevin