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

[Anthony Liguori <anthony@codemonkey.ws>]

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.

Regards,

Anthony Liguori

> ---
>   block/raw-posix.c |    5 +++++
>   1 files changed, 5 insertions(+), 0 deletions(-)
>
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 291699f..0ea79b6 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -1139,6 +1139,11 @@ static int cdrom_open(BlockDriverState *bs, const char *filename, int flags)
>       BDRVRawState *s = bs->opaque;
>
>       s->type = FTYPE_CD;
> +    if (flags&  BDRV_O_NOCACHE) {
> +        fprintf(stderr, "Disabling unsupported O_DIRECT (cache=none) for "
> +                "CDROM/DVD device (%s)\n", filename);
> +        flags&= ~BDRV_O_NOCACHE;
> +    }
>
>       /* open will not fail even if no CD is inserted, so add O_NONBLOCK */
>       return raw_open_common(bs, filename, flags, O_NONBLOCK);
>