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

[Anthony Liguori]

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, address@hidden wrote:
> >      
> > > From: Jes Sorensen<address@hidden>
> > >
> > > 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<address@hidden>
> > >
> > >        
> > 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
>