Re: [BUG] blkback reporting incorrect number of sectors, unable to boot

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@lists.xen.org] On Behalf Of
> Roger Pau Monné
> Sent: 07 November 2017 10:30
> To: Jan Beulich <JBeulich@suse.com>
> Cc: Mike Reardon <mule@inso.org>; xen-devel@lists.xen.org; Konrad
> Rzeszutek Wilk <konrad.wilk@oracle.com>
> Subject: Re: [Xen-devel] [BUG] blkback reporting incorrect number of
> sectors, unable to boot
> 
> On Mon, Nov 06, 2017 at 05:33:37AM -0700, Jan Beulich wrote:
> > >>> On 04.11.17 at 05:48, <mule@inso.org> wrote:
> > > I added some additional storage to my server with some native 4k sector
> > > size disks.  The LVM volumes on that array seem to work fine when
> mounted
> > > by the host, and when passed through to any of the Linux guests, but
> > > Windows guests aren't able to use them when using PV drivers.  The
> work
> > > fine to install when I first install Windows (Windows 10, latest build) but
> > > once I install the PV drivers it will no longer boot and give an
> > > inaccessible boot device error.  If I assign the storage to a different
> > > Windows guest that already has the drivers installed (as secondary
> storage,
> > > not as the boot device) I see the disk listed in disk management, but the
> > > size of the disk is 8x larger than it should be.  After looking into it a
> > > bit, the disk is reporting 8x the number of sectors it should have when I
> > > run xenstore-ls.  Here is the info from xenstore-ls for the relevant
> volume:
> > >
> > >       51712 = ""
> > >        frontend = "/local/domain/8/device/vbd/51712"
> > >        params = "/dev/tv_storage/main-storage"
> > >        script = "/etc/xen/scripts/block"
> > >        frontend-id = "8"
> > >        online = "1"
> > >        removable = "0"
> > >        bootable = "1"
> > >        state = "2"
> > >        dev = "xvda"
> > >        type = "phy"
> > >        mode = "w"
> > >        device-type = "disk"
> > >        discard-enable = "1"
> > >        feature-max-indirect-segments = "256"
> > >        multi-queue-max-queues = "12"
> > >        max-ring-page-order = "4"
> > >        physical-device = "fe:0"
> > >        physical-device-path = "/dev/dm-0"
> > >        hotplug-status = "connected"
> > >        feature-flush-cache = "1"
> > >        feature-discard = "0"
> > >        feature-barrier = "1"
> > >        feature-persistent = "1"
> > >        sectors = "34359738368"
> > >        info = "0"
> > >        sector-size = "4096"
> > >        physical-sector-size = "4096"
> > >
> > >
> > > Here are the numbers for the volume as reported by fdisk:
> > >
> > > Disk /dev/tv_storage/main-storage: 16 TiB, 17592186044416 bytes,
> 4294967296
> > > sectors
> > > Units: sectors of 1 * 4096 = 4096 bytes
> > > Sector size (logical/physical): 4096 bytes / 4096 bytes
> > > I/O size (minimum/optimal): 4096 bytes / 4096 bytes
> > > Disklabel type: dos
> > > Disk identifier: 0x00000000
> > >
> > > Device                        Boot Start        End    Sectors Size Id Type
> > > /dev/tv_storage/main-storage1          1 4294967295 4294967295  16T ee
> GPT
> > >
> > >
> > > As with the size reported in Windows disk management, the number of
> sectors
> > > from xenstore seems is 8x higher than what it should be.  The disks aren't
> > > using 512b sector emulation, they are natively 4k, so I have no idea where
> > > the 8x increase is coming from.
> >
> > Hmm, looks like a backend problem indeed: struct hd_struct's
> > nr_sects (which get_capacity() returns) looks to be in 512-byte
> > units, regardless of actual sector size. Hence the plain
> > get_capacity() use as well the (wrongly open coded) use of
> > part_nr_sects_read() looks insufficient in vbd_sz(). Roger,
> > Konrad?
> 
> Hm, AFAICT sector-size should always be set to 512.
> 
> > Question of course is whether the Linux frontend then
> > also needs adjustment, and hence whether the backend can
> > be corrected in a compatible way in the first place.
> 
> blkfront uses set_capacity, which also seems to expect the sectors to
> be hardcoded to 512.
> 

Oh dear. No wonder it's all quite broken.

  Paul

> Roger.
> 
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