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Date: Mon, 12 Oct 2009 20:15:43 +0400
From: Michael Tokarev <mjt@tls.msk.ru>
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Subject: Re: [Qemu-devel] [PATCH] Add readonly flag to -drive command
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Naphtali Sprei <nsprei@redhat.com>, qemu-devel@nongnu.org

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.

/mjt