[Qemu-devel] Re: [PATCH 3/3] raw-posix: Re-open host CD-ROM after media change

[Juan Quintela]

Stefan Hajnoczi <address@hidden> wrote:
> Piggy-back on the guest CD-ROM polling to poll on the host.  Open and
> close the host CD-ROM file descriptor to ensure we read the new size and
> not a stale size.
>
> Two things are going on here:
>
> 1. If hald/udisks is not already polling CD-ROMs on the host then
>    re-opening the CD-ROM causes the host to read the new medium's size.
>
> 2. There is a bug in Linux which means the CD-ROM file descriptor must
>    be re-opened in order for lseek(2) to see the new size.  The
>    inode size gets out of sync with the underlying device (which you can
>    confirm by checking that /sys/block/sr0/size and lseek(2) do not
>    match after media change).  I have raised this with the
>    maintainers but we need a workaround for the foreseeable future.
>
> Note that these changes are all in a #ifdef __linux__ section.
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  block/raw-posix.c |   25 +++++++++++++++++++++----
>  1 files changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index a95c8d4..ac95467 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -1240,10 +1240,27 @@ static int cdrom_is_inserted(BlockDriverState *bs)
>      BDRVRawState *s = bs->opaque;
>      int ret;
>  
> -    ret = ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT);
> -    if (ret == CDS_DISC_OK)
> -        return 1;
> -    return 0;
> +    /*
> +     * Opening and closing on each drive status check ensures the medium size
> +     * is refreshed.  If the file descriptor is kept open the size can become
> +     * stale.  This is essentially replicating CD-ROM polling but is driven 
> by
> +     * the guest.  As the guest polls, we poll the host.
> +     */

Comment confused me :p

we are not opening and closing at each status check.
We are opening if the cdrom is _not_ opened.  And we are closing if
there is one error.  If comment 2 above is right, you need to do
insteand something like:

        if (s->fd != -1) {
           close(s->fd);
        }

        s->fd = open( ....);

That is really reopening all the times.

> +
> +    if (s->fd == -1) {
> +        s->fd = qemu_open(bs->filename, s->open_flags, 0644);

Everything else on that file uses plain "open" not "qemu_open".
diference is basically that qemu_open() adds flag O_CLOEXEC.

I don't know if this one should be vanilla open or the other ones
qemu_open().

What do you think?


> +        if (s->fd < 0) {
> +            return 0;
> +        }
> +    }
> +
> +    ret = (ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT) == CDS_DISC_OK);

parens are not needed around ==.

> +
> +    if (!ret) {
> +        close(s->fd);
> +        s->fd = -1;
> +    }
> +    return ret;
>  }
>  
>  static int cdrom_eject(BlockDriverState *bs, int eject_flag)

Later, Juan.