Linux domU only works with xen_platform_pci=0 ?

[Kai Otto]

Hello,

I'm trying to set up a FreeBSD 11.1 system as Xen virtualization host.
Following a combination of handbook [1] and wiki [2], I was able to get
get a FreeBSD dom0 in PVH mode, and FreeBSD domU in HVM mode running,
including installation to disk and networking.

It seemed to me as if the switch 'xen_platform_pci' doesn't have an
effect on FreeBSD domU's, as I see e.g. a xenpci0 and xbd0 in dmesg no
matter if I set it to 1 or 0.

Do I understand it correctly, that this switch makes the difference
between HVM and PVHVM mode?
According to xl.cfg(5), it 'enables a guest Operating System [...] to
make use of paravirtualization features such as disk and network devices'.


Afterwards, I tried to create a Linux domU. Both Centos 7 and Alpine
Linux only detected the harddisk with xen_platform_pci=0.

For Alpine Linux, with xen_platform_pci=1, I get the following messages
on the console:

vbd vbd-5632: 19 xenbus_dev_probe on device/vbd/5632
vbd vbd-5632: failed to write error node for device/vbd/5632 (19
xenbus_dev_probe on device/vbd/5632)

After waiting for a couple minutes it boots, but doesn't detect the disk.

Looking in /var/log/xen/qemu-dm-alpine-hvm.log, I see the following
messages:

xen be core: can't open gnttab device
xen be core: can't open gnttab device
xen be: vkbd-0: initalize() failed
xen be: vkbd-0: initalize() failed
xen be: vkbd-0: initalize() failed



Googling around yielded [3], where someone apparently ran into the same
problem, and used xen_platform_pci=0 as workaround.



So my question is:
Is xen_platform_pci=1 required for PVHVM mode?
If yes, is PVHVM mode only available for FreeBSD domU's?
If no, how can I enable it for Linux guests?


Any response would be appreciated, thanks!

(Unrelated: I also couldn't get PVH guests to boot, neither FreeBSD nor
Linux. The console stayed blank and CPU went to 100%. But that's a whole
other story I guess.)

- Kai




My system:

OS: FreeBSD-11.1-RELEASE amd64
xen installed via pkg
Processor: E3-1240 v2, (which supports VT-x, VT-d and EPT)


My alpine.cfg:

builder="hvm"
name="alpine-hvm"
memory=1024
vcpus=1
vif = [ 'bridge=bridge0' ]
disk = [ '/dev/zvol/zbulk/vm/alpine-hvm_disk0,raw,hda,rw',
 '/zbulk/mediashare/isos/linux/apline-virt-3.7.0-x86_64.iso,raw,hdb:cdrom,r']

xen_platform_pci=1 # or 0, see above
vnc = 1
vnclisten = "0.0.0.0"
serial = "pty"
usbdevice = "tablet"

-- 

[1]
https://www.freebsd.org/doc/en/books/handbook/virtualization-host-xen.html
[2] https://wiki.freebsd.org/Xen
[3]
https://www.reddit.com/r/freebsd/comments/6uldys/freebsd_111_linux_domu_on_xen_wont_boot/