Re: [PATCH 11/12] hw/xen: automatically assign device index to block devices

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2023-10-18 at 09:32 +0200, Igor Mammedov wrote:
> On Mon, 16 Oct 2023 16:19:08 +0100
> David Woodhouse <dwmw2@infradead.org> wrote:
> 
> > From: David Woodhouse <dwmw@amazon.co.uk>
> > 
> 
> is this index a user (guest) visible?

Yes. It defines what block device (e.g. /dev/xvda) the disk appears as
in the guest. In the common case, it literally encodes the Linux
major/minor numbers. So xvda (major 202) is 0xca00, xvdb is 0xca10 etc.

Previously we had to explicitly set it for each disk in Qemu:

  -drive file=disk1.img,id=drv1 -device xen-disk,drive=drv1,vdev=xvda
  -drive file=disk2.img,id=drv2 -device xen-disk,drive=drv2,vdev=xvdb

Now we can just do

  -drive file=disk1.img,if=xen -drive file-disk2.img,if=xen

(We could go further and make if=xen the default for Xen guests too,
but that doesn't work right now because xen-block will barf on the
default provided CD-ROM when it's empty. It doesn't handle empty
devices. And if I work around that, then `-hda disk1.img` would work on
the command line... but would make it appear as /dev/xvda instead of
/dev/hda, and I don't know how I feel about that.)

[root@localhost ~]# xenstore-ls  -f device/vbd
device/vbd = ""
device/vbd/51712 = ""
device/vbd/51712/backend = "/local/domain/0/backend/qdisk/1/51712"
device/vbd/51712/backend-id = "0"
device/vbd/51712/device-type = "disk"
device/vbd/51712/event-channel = "8"
device/vbd/51712/feature-persistent = "1"
device/vbd/51712/protocol = "x86_64-abi"
device/vbd/51712/ring-ref = "8"
device/vbd/51712/state = "4"
device/vbd/51712/virtual-device = "51712"

> 
> > There's no need to force the user to assign a vdev. We can automatically
> > assign one, starting at xvda and searching until we find the first disk
> > name that's unused.
> > 
> > This means we can now allow '-drive if=xen,file=xxx' to work without an
> > explicit separate -driver argument, just like if=virtio.
> > 
> > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> > ---
> >  blockdev.c           | 15 ++++++++++++---
> >  hw/block/xen-block.c | 25 +++++++++++++++++++++++++
> >  2 files changed, 37 insertions(+), 3 deletions(-)
> > 
> > diff --git a/blockdev.c b/blockdev.c
> > index 325b7a3bef..9dec4c9c74 100644
> > --- a/blockdev.c
> > +++ b/blockdev.c
> > @@ -255,13 +255,13 @@ void drive_check_orphaned(void)
> >           * Ignore default drives, because we create certain default
> >           * drives unconditionally, then leave them unclaimed.  Not the
> >           * users fault.
> > -         * Ignore IF_VIRTIO, because it gets desugared into -device,
> > -         * so we can leave failing to -device.
> > +         * Ignore IF_VIRTIO or IF_XEN, because it gets desugared into
> > +         * -device, so we can leave failing to -device.
> >           * Ignore IF_NONE, because leaving unclaimed IF_NONE remains
> >           * available for device_add is a feature.
> >           */
> >          if (dinfo->is_default || dinfo->type == IF_VIRTIO
> > -            || dinfo->type == IF_NONE) {
> > +            || dinfo->type == IF_XEN || dinfo->type == IF_NONE) {
> >              continue;
> >          }
> >          if (!blk_get_attached_dev(blk)) {
> > @@ -977,6 +977,15 @@ DriveInfo *drive_new(QemuOpts *all_opts, BlockInterfaceType block_default_type,
> >          qemu_opt_set(devopts, "driver", "virtio-blk", &error_abort);
> >          qemu_opt_set(devopts, "drive", qdict_get_str(bs_opts, "id"),
> >                       &error_abort);
> > +    } else if (type == IF_XEN) {
> > +        QemuOpts *devopts;
> > +        devopts = qemu_opts_create(qemu_find_opts("device"), NULL, 0,
> > +                                   &error_abort);
> > +        qemu_opt_set(devopts, "driver",
> > +                     (media == MEDIA_CDROM) ? "xen-cdrom" : "xen-disk",
> > +                     &error_abort);
> > +        qemu_opt_set(devopts, "drive", qdict_get_str(bs_opts, "id"),
> > +                     &error_abort);
> >      }
> >  
> >      filename = qemu_opt_get(legacy_opts, "file");
> > diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
> > index 9262338535..20fa783cbe 100644
> > --- a/hw/block/xen-block.c
> > +++ b/hw/block/xen-block.c
> > @@ -34,6 +34,31 @@ static char *xen_block_get_name(XenDevice *xendev, Error **errp)
> >      XenBlockDevice *blockdev = XEN_BLOCK_DEVICE(xendev);
> >      XenBlockVdev *vdev = &blockdev->props.vdev;
> >  
> > +    if (blockdev->props.vdev.type == XEN_BLOCK_VDEV_TYPE_INVALID) {
> > +        char name[11];
> > +        int disk = 0;
> > +        unsigned long idx;
> > +
> > +        /* Find an unoccupied device name */
> > +        while (disk < (1 << 20)) {
> > +            if (disk < (1 << 4)) {
> > +                idx = (202 << 8) | (disk << 4);
> > +            } else {
> > +                idx = (1 << 28) | (disk << 8);
> > +            }
> > +            snprintf(name, sizeof(name), "%lu", idx);
> > +            if (!xen_backend_exists("qdisk", name)) {
> > +                vdev->type = XEN_BLOCK_VDEV_TYPE_XVD;
> > +                vdev->partition = 0;
> > +                vdev->disk = disk;
> > +                vdev->number = idx;
> > +                return g_strdup(name);
> > +            }
> > +            disk++;
> > +        }
> > +        error_setg(errp, "cannot find device vdev for block device");
> > +        return NULL;
> > +    }
> >      return g_strdup_printf("%lu", vdev->number);
> >  }
> >  
> 
> 


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