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

On 31/01/14 00:50, Kuba wrote:
Dear List,

I am trying to set up a following configuration:
1. very simple Linux-based dom0 (Debian 7.3) with Xen 4.3.1 compiled from sources, 2. one storage VM (FreeBSD 10, HVM+PV) with SATA controller attached using VT-d, exporting block devices via iSCSI to other VMs and physical machines, 3. one Windows 7 SP1 64 VM (HVM+GPLPV) with GPU passthrough (Quadro 4000) installed on a block device exported from the storage VM (target 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 installation, Windows needs to reboot, boots fine, displays a message that PV SCSI drivers were installed and needs to reboot again, and then cannot boot. Sometimes it gets stuck at "booting from harddrive" in SeaBIOS, sometimes BSODs with "unmountable boot volume" message. All of the following I tried without GPU passthrough to narrow down the problem.

The intriguing part is this:

1. If the storage VM's OS is Linux - it fails with the above symptoms.
2. If the block devices for the storage VM come directly from dom0 (not 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 all works.
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, without 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 hard drive as a block device to the storage VM and re-export it through iSCSI, Windows usually boots fine, but works unstable. And by unstable I mean random read/write errors, sometimes programs won't start, ntdll.dll 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 where the 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 itself only in such a particular configuration (hope not)?

IMHO, it sounds like a resource issue... the domU providing the iSCSI, plus the dom0 plus the domU (windows VM) are all asking for CPU, IRQ's, etc, and someone isn't getting enough in time. It doesn't really help, but we use a physical iSCSI server, the dom0 then connects to the iSCSI and provides the disks to the VM's. Maybe look at assigning specific exclusive CPU's to each dom0 and domU's, and see if you can still reproduce the issue. Also, make absolutely sure that you don't have two VM's accessing the same iSCSI.


Adam Goryachev Website Managers www.websitemanagers.com.au

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