Re: [Qemu-devel] [PATCH] raw-posix.c: remove raw device access for cdrom

[Stefan Hajnoczi]

On Wed, Jul 1, 2015 at 11:13 PM, Programmingkid
<address@hidden> wrote:
> Fix real cdrom access in Mac OS X so it can be used in QEMU.
> It simply removes the r from a device file's name. This
> allows for a real cdrom to be accessible to the guest.
> It has been successfully tested with a Windows XP guest
> in qemu-system-i386. The qemu-system-ppc emulator doesn't
> quit anymore, but there is another problem that prevents a
> real cdrom from working.
>
> Signed-off-by: John Arbuckle <address@hidden>
>
> ---
>  block/raw-posix.c |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index a967464..3585ed9 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -2096,6 +2096,16 @@ static int hdev_open(BlockDriverState *bs, QDict
> *options, int flags,
>          kernResult = FindEjectableCDMedia( &mediaIterator );
>          kernResult = GetBSDPath( mediaIterator, bsdPath, sizeof( bsdPath )
> );
>
>
>
> +        /*
> +         * Remove the r from cdrom block device if needed.
> +         * /dev/rdisk1 would become /dev/disk1.
> +         * The r means raw access. It doesn't work well.
> +         */
> +        int sizeOfString = strlen("/dev/r");
> +        if (strncmp("/dev/r", bsdPath, sizeOfString) == 0) {
> +            sprintf(bsdPath, "/dev/%s", bsdPath + sizeOfString);
> +        }

Why doesn't raw access "work well"?

This patch looks like a workaround but you haven't identified the root cause.

Perhaps because of request alignment requirements?  Typically CD-ROMs
have 2 KB block size.  You could test this by changing the following
in block_int.h:
#define BLOCK_PROBE_BUF_SIZE        512

In that case you need to fix raw-posix.c alignment detection code for
Mac OS X.  Look at raw_probe_alignment().

Stefan