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Date: Tue, 16 Jun 2009 20:58:49 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] Regression opening read-only cdroms
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To: Jamie Lokier <jamie@shareable.org>
Cc: Christoph Hellwig <hch@lst.de>, qemu-devel <qemu-devel@nongnu.org>

On 06/16/2009 08:51 PM, Jamie Lokier wrote:
> Avi Kivity wrote:
>    
>> On 06/16/2009 06:54 PM, Jamie Lokier wrote:
>>      
>>> read-only disk images don't make much sense.
>>>
>>>
>>> And yet "chmod 444 image; qemu ..." works.
>>> If you're booting from a disk you don't need to write to, obviously.
>>> Generally it'll need to be mounted read-only in the guest.
>>>
>>>        
>> It will eventually fail.  Open the ext3 log, update atime, or
>> something.  The guest expects the disk to be writeable.
>>      
>
> No.  Obviously if you _want_ to run a guest with the disk mounted
> writable, you'll use snapshot=on instead because that's what it's for.
>
> Otherwise, a read-only disk should works fine using virtio/SCSI/USB,
> as the guest will mount it read-only, as those interfaces all have a
> read-only media flag which Linux guests (at least) look at.
>
> Which is the desired behaviour.
>
>    

I didn't consider a read-only media flag.  I retract my remarks, -drive 
...,readonly then makes sense for those interfaces.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.