qemu-nbd: Deprecate qemu-nbd --partition

The existing qemu-nbd --partition code claims to handle logical
partitions up to 8, since its introduction in 2008 (commit 7a5ca86).
However, the implementation is bogus (actual MBR logical partitions
form a sort of linked list, with one partition per extended table
entry, rather than four logical partitions in a single extended
table), making the code unlikely to work for anything beyond -P5 on
actual guest images. What's more, the code does not support GPT
partitions, which are becoming more popular, and maintaining device
subsetting in both NBD and the raw device is unnecessary duplication
of effort (even if it is not too difficult).

Note that obtaining the offsets of a partition (MBR or GPT) can be
learned by using 'qemu-nbd -c /dev/nbd0 file.qcow2 && sfdisk --dump
/dev/nbd0', but by the time you've done that, you might as well
just mount /dev/nbd0p1 that the kernel creates for you instead of
bothering with qemu exporting a subset.  Or, keeping to just
user-space code, use nbdkit's partition filter, which has already
known both GPT and primary MBR partitions for a while, and was
just recently enhanced to support arbitrary logical MBR parititions.

Start the clock on the deprecation cycle, with examples of how
to accomplish device subsetting without using -P.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190125234837.2272-1-eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

qemu-deprecated.texi

@@ -160,3 +160,36 @@ Example of legacy encoding:
 The above, converted to the current supported format:
 
 @code{json:@{"file.driver":"rbd", "file.pool":"rbd", "file.image":"name"@}}
+
+@section Related binaries
+
+@subsection qemu-nbd --partition (since 4.0.0)
+
+The ``qemu-nbd --partition $digit'' code (also spelled @option{-P})
+can only handle MBR partitions, and has never correctly handled
+logical partitions beyond partition 5.  If you know the offset and
+length of the partition (perhaps by using @code{sfdisk} within the
+guest), you can achieve the effect of exporting just that subset of
+the disk by use of the @option{--image-opts} option with a raw
+blockdev using the @code{offset} and @code{size} parameters layered on
+top of any other existing blockdev. For example, if partition 1 is
+100MiB long starting at 1MiB, the old command:
+
+@code{qemu-nbd -t -P 1 -f qcow2 file.qcow2}
+
+can be rewritten as:
+
+@code{qemu-nbd -t --image-opts driver=raw,offset=1M,size=100M,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file.qcow2}
+
+Alternatively, the @code{nbdkit} project provides a more powerful
+partition filter on top of its nbd plugin, which can be used to select
+an arbitrary MBR or GPT partition on top of any other full-image NBD
+export.  Using this to rewrite the above example results in:
+
+@code{qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &}
+@code{nbdkit -f --filter=partition nbd socket=/tmp/sock partition=1}
+
+Note that if you are exposing the export via /dev/nbd0, it is easier
+to just export the entire image and then mount only /dev/nbd0p1 than
+it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a
+subset of the image.

qemu-nbd.c

@@ -787,6 +787,8 @@ int main(int argc, char **argv)
             flags &= ~BDRV_O_RDWR;
             break;
         case 'P':
+            warn_report("The '-P' option is deprecated; use --image-opts with "
+                        "a raw device wrapper for subset exports instead");
             if (qemu_strtoi(optarg, NULL, 0, &partition) < 0 ||
                 partition < 1 || partition > 8) {
                 error_report("Invalid partition '%s'", optarg);

qemu-nbd.texi

@@ -56,8 +56,10 @@ auto-detecting.
 @item -r, --read-only
 Export the disk as read-only.
 @item -P, --partition=@var{num}
-Only expose MBR partition @var{num}.  Understands physical partitions
-1-4 and logical partitions 5-8.
+Deprecated: Only expose MBR partition @var{num}.  Understands physical
+partitions 1-4 and logical partition 5. New code should instead use
+@option{--image-opts} with the raw driver wrapping a subset of the
+original image.
 @item -B, --bitmap=@var{name}
 If @var{filename} has a qcow2 persistent bitmap @var{name}, expose
 that bitmap via the ``qemu:dirty-bitmap:@var{name}'' context