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Re: [Xen-devel] [PATCH v2 14/18] xen: add implementations of xen-block connect and disconnect functions...

On Thu, Dec 06, 2018 at 03:08:40PM +0000, Paul Durrant wrote:
> ...and wire in the dataplane.
> This patch adds the remaining code to make the xen-block XenDevice
> functional. The parameters that a block frontend expects to find are
> populated in the backend xenstore area, and the 'ring-ref' and
> 'event-channel' values specified in the frontend xenstore area are
> mapped/bound and used to set up the dataplane.
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

With this patch, we should be able to have QEMU instantiate a new
backend for a guest, right ? (via command line or QMP)

I've tried, and that doesn't work, the xenstore path for the frontend is
wrong. In the qemu trace, I have:
    xs_node_create /local/domain/0/backend/xen-disk/23/268572709
Which is probably fine, even if not described in xenstore-paths.markdown.
    xs_node_create /local/domain/23/device/xen-disk/268572709
Which is not, instead of "xen-disk", we should have "vbd".

I know that this is fixed in "xen: automatically create
XenBlockDevice-s", but at least the "vbd" type couldn't be added in this
patch, or maybe a previous one.

Another issue seems to be error handling. I've done a very simple test,
I've added '-device xen-disk,vdev=d536p37,id=mydisk' to the command
line (which is obvious wrong), and QEMU abort with:
    qemu-system-i386: hw/block/xen-block.c:174: xen_block_realize: Assertion 
`conf->blk' failed.
But I've pointed out the error in the code below.

And just for fun, adding then removing a xen-disk via QMP. Adding works
fine (once I've fixed the frontend name). I've run the following with
blockdev-add driver=file  filename=/root/vm/disk/testing-disk.qcow2 
blockdev-add driver=qcow2 node-name=emptyqcow2 file=emptyfile
device_add driver=xen-disk vdev=xvdn id=fromqmp drive=emptyqcow2

But, then, remove doesn't work, running "device_del id=fromqmp" doesn't
do anything. I guess we can try to fix that later if you don't find
what's missing.

> @@ -76,6 +151,7 @@ static void xen_block_realize(XenDevice *xendev, Error 
> **errp)
>      const char *type = object_get_typename(OBJECT(blockdev));
>      XenBlockVdev *vdev = &blockdev->vdev;
>      Error *local_err = NULL;
> +    BlockConf *conf = &blockdev->conf;
>      if (vdev->type == XEN_BLOCK_VDEV_TYPE_INVALID) {
>          error_setg(errp, "vdev property not set");
> @@ -90,6 +166,59 @@ static void xen_block_realize(XenDevice *xendev, Error 
> **errp)
>              error_propagate(errp, local_err);

You probably want to add a return here, this is when
`blockdev_class->realize' fails.

>          }
>      }
> +
> +    /*
> +     * The blkif protocol does not deal with removable media, so it must
> +     * always be present, even for CDRom devices.
> +     */
> +    assert(conf->blk);

That assert should probably not be there, as a missing conf->blk isn't a
programming error, but a user error, I think.

Actually, the issue is the missing return abrove, and the assert is
probably fine.

Anthony PERARD

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