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Re: [Xen-users] Strange failures of Xen 4.3.1, PVHVM storage VM, iSCSI and Windows+GPLPV VM combination

  • To: xen-users@xxxxxxxxxxxxx
  • From: Kuba <kuba.0000@xxxxx>
  • Date: Sun, 09 Feb 2014 12:47:19 +0100
  • Delivery-date: Sun, 09 Feb 2014 11:47:53 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

W dniu 2014-02-07 10:25, Roger Pau Monnà pisze:
On 06/02/14 23:32, Kuba wrote:
W dniu 2014-02-06 09:22, Roger Pau Monnà pisze:
On 05/02/14 22:54, Kuba wrote:
W dniu 2014-02-05 17:54, Roger Pau Monnà pisze:
On 05/02/14 17:43, Kuba wrote:
W dniu 2014-02-05 17:29, Roger Pau Monnà pisze:
On 05/02/14 17:13, Kuba wrote:
W dniu 2014-02-01 20:27, Kuba pisze:
W dniu 2014-01-31 02:35, James Harper pisze:

I am trying to set up a following configuration:
1. very simple Linux-based dom0 (Debian 7.3) with Xen 4.3.1
from sources,
2. one storage VM (FreeBSD 10, HVM+PV) with SATA controller
using VT-d, exporting block devices via iSCSI to other VMs and
3. one Windows 7 SP1 64 VM (HVM+GPLPV) with GPU passthrough
4000) installed on a block device exported from the storage VM
on the storage VM, initiator on dom0).

Everything works perfectly (including PCI & GPU passthrough)
until I
install GPLPV drivers on the Windows VM. After driver
Windows needs to reboot, boots fine, displays a message that PV


drivers were installed and needs to reboot again, and then cannot
Sometimes it gets stuck at "booting from harddrive" in SeaBIOS,
sometimes BSODs with "unmountable boot volume" message. All of
following I tried without GPU passthrough to narrow down the

The intriguing part is this:

1. If the storage VM's OS is Linux - it fails with the above
2. If the block devices for the storage VM come directly from
via pci-passthrough) - it fails.
2. If the storage VM is an HVM without PV drivers (e.g. FreeBSD
9.2-GENERIC) - it all works.
3. If the storage VM's OS is Linux with kernel compiled
without Xen
guest support - it works, but is unstable (see below).
4. If the iSCSI target is on a different physical machine - it
5. If the iSCSI target is on dom0 itself - it works.
6. If I attach the AHCI controller to the Windows VM and install
directly on the hard drive - it works.
7. If the block device for Windows VM is a disk, partition,
file, LVM
volume or even a ZoL's zvol (and it comes from a dom0 itself,
iSCSI)- it works.

If I install Windows and the GPLPV drivers on a hard drive
attached to
dom0, Windows + GPLPV work perfectly. If I then give the same
as a block device to the storage VM and re-export it through


Windows usually boots fine, but works unstable. And by unstable I
random read/write errors, sometimes programs won't start,
crashes, and after couple reboots Windows won't boot (just like
mentioned above).

The configurations I would like to achieve makes sense only
with PV
drivers on both storage and Windows VM. All of the "components"
seem to
work perfectly until all put together, so I am not really sure
problem is.

I would be very grateful for any suggestions or ideas that could
possibly help to narrow down the problem. Maybe I am just doing
something wrong (I hope so). Or maybe there is a bug that shows
only in such a particular configuration (hope not)?

I'm curious about prompting for the pvscsi drivers to be
installed. Is
this definitely what it is asking for? Pvscsi for gplpv is
removed in
the latest versions and suffered varying degrees of bitrot in
versions. If you have the iscsi initiator in dom0 then exporting a
block device to windows via the normal vbd channel should be just

You've gone to great lengths to explain the various things you've
tried, but I think I'm a little confused on where the iscsi
is in the "doesn't work" scenarios. I'm having a bit of an off day
today so it's probably just me, but above I have highlighted
the two
scenarios... could you fill me in on a few things:

At (a) and (b), is the iscsi initiator in dom0, or are you
booting windows directly via iscsi?

At (b), with latest debug build of gplpv, can you run debugview
sysinternals.com and see if any interesting messages are displayed
before things fall in a heap?

Are any strange logs shown in any of Win DomU, Dom0, or storage

How big are your disks?

Can you reproduce with only one vcpu?

What bridge are you using? Openvswitch or traditional linux

What MTU are you using on your storage network? If you are using
frames can you go back to 1500 (or at least <= 4000)?

Can you turn off scatter gather, Large Send Offload (GSO), and IP
Checksum offload on all the iscsi endpoints?

Can you turn on data digest/checksum on iscsi? If all endpoints
support it then this would provide additional verification that
of the network packets are getting corrupted.

Would driver domain work in your scenario? Then the disk could be
attached directly from your storage DomU without accruing all the
iscsi overhead. I'm not up with the status of HVM, vbd, and driver
domain so I don't know if this is possible.

More questions than answers. Sorry :)


Dear James,

thank you for your questions - I really appreciate everything that
help me move closer to solving or isolating the problem.

I'll check what type of driver is used exactly - up until now I
just installed all drivers included in the package, I thought
all of
them were necessary. I'll try installing them without XenScsi.

Do you mean revisions > 1092:85b99b9795a6 by "the latest versions"?
Which version should I use?

Forgive me if the descriptions were unclear. The initiator was
always in
dom0. I only moved the target to dom0 or a separate physical
machine in
(4) and (5). I didn't boot Windows directly from iSCSI (in fact I
couple times, but had some problems with it, so I didn't mention

My "disks" (the block devices I dedicated to the Windows VM) were
120GB and 240GB SSDs, ~100GB ZVOLs and 50GB LVM volumes.

I'm using traditional linux bridge. I didn't set MTUs explicitly,
so I
assume it's 1500, but I will verify this.

I'd love to use a storage driver domain, but the wiki says "It
is not
possible to use driver domains with pygrub or HVM guests yet". But
page is a couple of months old, maybe it's an outdated info? It
is worth checking out.

I'll do my best to provide answers to the remaining questions as
soon as
possible. Thank you for so many ideas.

Best regards,

Xen-users mailing list

It seems the problems are not related to GPLPV. There is an easy
way to
reproduce the issues without Windows and without installing
using only livecds for two DomUs:

1) Set up a Linux Dom0 with Xen 4.3.1 and standard Linux bridge for
and DomUs

Are you using a Xen build with debugging enabled? I think I might
have a
clue of what's happening, because I also saw it. Could you recompile
with debugging enabled and try the same test (iSCSI target on DomU
initiator on Dom0)?


Xen-users mailing list

Of course I could! Please point me to any relevant information on
how to
build Xen with debugging enabled and what to do next. I build Xen
standard ./configure && make world && make install.

Just `make debug=y xen` and boot with the resulting xen.gz.


Xen-users mailing list

I ran the test using debug build of Xen. This time I gave the name "tgt"
to the DomU with iSCSI target, and the other domain was named simply
"domu". Sorry for the inconsistency. After logging in to the iSCSI
target from Dom0, I ran "mkfs.ext4 /dev/sdb" (still in Dom0). So far, so
good. Then I launched the other DomU and as soon as I executed
"fsck.ext4 /dev/xvda", some errors appeared in the output of "xl dmesg"
(attached as "xl-dmesg.log"). Surprisingly, the first fsck succeeded.
Unfortunately, executing fsck.ext4 for the second time showed serious
file system errors. The fsck commands were the only things I ran that
touched /dev/xvda. After shutting down "domu", when I tried to log out
from the iSCSI target, an error came up in Dom0's dmesg
("dom0-dmesg.log"). Logs from /var/log/xen/ are also attached.

I will happily run next tests - just tell me what can I do :)


This is the same problem I've seen when using a similar setup. The root
of the problem is that blkback maps a grant ref to a memory page in
Dom0, then this memory page ends up in netback, and when netback tries
to issue a GNTTABOP_copy using the mfn of this grant mapped page the
operation fails because Xen detects that the mfn passed doesn't belong
to the guest.

The only way I can think of solving this is that netback detects that
the page is not local and somehow we use it's grant ref instead of mfn
(this means we would need to store the grant ref somewhere in the page).


As this is something far beyond my ability to solve, I couldn't resist
to try something else - running FreeBSD 10 as a storage driver domain. I
was able to provide a block device (zvol) from one FreeBSD DomU directly
to another FreeBSD DomU just like described in the wiki (with Qemu
traditional in the second DomU) and install the OS on it. Unfortunately
the second DomU's bios was unable to detect this "disk" and boot from it.

But with this command:
xl block-attach Domain-0

I was able to attach a block device exported from a DomU to Dom0 without
iSCSI and then using it as a disk for a second DomU (with
disk=['phy:/dev/xvds,xvda,w']). Now, if the second DomU had no PV
drivers (e.g. Windows without GPLPV), everything worked fine. But
running an OS with PV drivers (Linux or Windows+GPLPV) in the second
DomU resulted in a very similar errors in xl dmesg like in the
previously attached logs (see the attachment).

Do I understand correctly that solving the issue you are pointing out
would also allow to use OSes like FreeBSD as storage driver domain for
other PV-enabled DomUs? That would be something!

And most importantly - is there anything I can do to help?


Thanks for testing this use-case also. I have not debugged it closely,
but I think what's happening here is that blkback in Dom0 grant maps a
page passed from the DomU, and then the blkfront instance on Dom0 tries
to use gnttab_grant_foreign_access_ref on that page and fails miserably
(because the page doesn't belong to Dom0).

The right way to fix this would be to make blkfront and blkback in the
respective guests connect directly instead of using Dom0 as a proxy.
Mainly the toolstack in Dom0 needs to know you are trying to attach a
disk from a driver domain and DTRT (attach the disk locally to Dom0 for
HVM access, but write the PV disk info in xenstore so that the guest
connects directly to the blkback in the driver domain instead of Dom0).
Are you interested in submitting a patch for libxl to fix this?


I am truly sorry, I wish I could, but I'm afraid my very limited knowledge in both Xen and low-level x86 inner workings makes it simply impossible for me to help in this way. I can help with testing, though I'm aware it's not the same.

However, if I understand correctly, the ability to provide a block device from a FreeBSD-based storage driver domain directly to another FreeBSD-based DomU indicates that the majority of work has already been done.

Best regards,

Xen-users mailing list



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