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

[Anthony Liguori <anthony@codemonkey.ws>]

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).
>>
>> Still missing passing the read only attribute to the guest, where possible. I don't know which device types supports
>> read only, and don't know how to pass this information to guests.
>>
>> Also not sure what to do when qemu cannot open the file as writeable. Currently it opens it as read only.
>> We might change it to give a warning or even an error.
>>
>> From 6e297da79a4c015555e3927e6d28744933a31ebe Mon Sep 17 00:00:00 2001
>> From: Naphtali Sprei <nsprei@redhat.com>
>> Date: Mon, 12 Oct 2009 14:25:25 +0200
>> Subject: [PATCH] Added readonly flag to -drive command.
>>  This enables sharing same image between guests, with readonly access.
>>  Implementaion mark the drive as read_only and changes the flags when actually opening the file.
>>     
>
> Is this enough? Basically none of the block drivers know that their
> image could be read-only, so we'll likely trigger some unexpected error
> cases there. For a simple write I guess we'll be okay (not sure if we'll
> return the right error code, though), but I have no idea what, say,
> savevm would do with a read-only image.
>   

Yes, this is why patches like this have been rejected in the past.  This 
concept needs to be plumbed to the device emulation so that a guest 
realizes it has a read-only disk.  Throwing away writes without telling 
the guest is a recipe for disaster.

Regards,

Anthony Liguori