[Qemu-devel] Re: [PATCH] Disable O_DIRECT for physical CDROM/DVD drives

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/20/2010 11:02 AM, Jes Sorensen wrote:
> On 07/20/10 17:40, Anthony Liguori wrote:
>    
>> On 07/20/2010 10:17 AM, Jes.Sorensen@redhat.com wrote:
>>      
>>> From: Jes Sorensen<Jes.Sorensen@redhat.com>
>>>
>>> O_DIRECT (cache=none) requires sector alignment, however the physical
>>> sector size of CDROM/DVD drives is 2048, as opposed to most disk
>>> devices which use 512. QEMU is hard coding 512 all over the place, so
>>> allowing O_DIRECT for CDROM/DVD devices does not work.
>>>
>>> Signed-off-by: Jes Sorensen<Jes.Sorensen@redhat.com>
>>>
>>>        
>> Wouldn't a better solution be to have a cdrom_read/cdrom_write hook that
>> did the appropriate bouncing?
>>
>> Silently disabling something a user explicitly asked for is not a good
>> option.  In the very least, it should error out entirely.
>>      
> I thought about this, but it would require basically fixing up or
> copying all of the pread/pwrite code to use the right block size. This
> is really more of a band-aid but it should be pretty safe.
>    

Please throw an error.  If a user explicitly asks for something, and we 
can provide it, we should not continue.  Changing it to something else 
is a bug.

That doesn't apply when we're changing a default value, but if the user 
asks for something, we should give it to them or fail.

Regards,

Anthony Liguori

> Cheers,
> Jes
>