[Xen-devel] Questions about PVHv2/HVMlite

[Gary R Hook <ghook@xxxxxxx>]

So I've been slogging through online docs and the code, trying to 
understand where things stand with PVH.
I think my primary questions are:
  1) How do I identify a PVHv2/HVMlite guest?
  2) Or, perhaps more importantly, what distinguishes said guest?
I've got Xen 4.9 unstable built/installed/booted, and am running 4.10 
kernels on my
dom0 and guests.

I've gotten a guest booted, and a basic Ubuntu 14.04 installed from a 
distro ISO onto a
raw disk (a logical volume). All good.

If I use the example file /etc/xen/example.hvm to define a simple guest 
(but no VGA:
nographic=1), I see that I have a qemu instance running, which I expect, 
along with some
threads:

root      8523     1  0 14:31 ?        00:00:03 
/usr/local/lib/xen/bin/qemu-system-i386 -xen-domid 17 -chardev socket....
root      8779     2  0 14:31 ?        00:00:00 [17.xvda-0]
root      8780     2  0 14:31 ?        00:00:00 [vif17.0-q0-gues]
root      8781     2  0 14:31 ?        00:00:00 [vif17.0-q0-deal]
root      8782     2  0 14:31 ?        00:00:00 [vif17.0-q1-gues]
root      8783     2  0 14:31 ?        00:00:00 [vif17.0-q1-deal]

All seems good. Now, I've read through the doc at
        https://wiki.xen.org/wiki/Xen_Linux_PV_on_HVM_drivers
and when I log into the above guest, and run dmesg | egrep -i 
'xen|front' I get this output:
[    0.000000] DMI: Xen HVM domU, BIOS 4.9-rc 04/25/2017
[    0.000000] Hypervisor detected: Xen
[    0.000000] Xen version 4.9.
[    0.000000] Xen Platform PCI: I/O protocol version 1
[    0.000000] Netfront and the Xen platform PCI driver have been 
compiled for this kernel: unplug emulated NICs.
[    0.000000] Blkfront and the Xen platform PCI driver have been 
compiled for this kernel: unplug emulated disks.
[    0.000000] ACPI: RSDP 0x00000000000F6800 000024 (v02 Xen   )
[    0.000000] ACPI: XSDT 0x00000000FC00A5B0 000054 (v01 Xen    HVM 
00000000 HVML 00000000)
[    0.000000] ACPI: FACP 0x00000000FC00A2D0 0000F4 (v04 Xen    HVM 
00000000 HVML 00000000)
[    0.000000] ACPI: DSDT 0x00000000FC0012A0 008FAC (v02 Xen    HVM 
00000000 INTL 20140214)
[    0.000000] ACPI: APIC 0x00000000FC00A3D0 000070 (v02 Xen    HVM 
00000000 HVML 00000000)
[    0.000000] ACPI: HPET 0x00000000FC00A4C0 000038 (v01 Xen    HVM 
00000000 HVML 00000000)
[    0.000000] ACPI: WAET 0x00000000FC00A500 000028 (v01 Xen    HVM 
00000000 HVML 00000000)
[    0.000000] ACPI: SSDT 0x00000000FC00A530 000031 (v02 Xen    HVM 
00000000 INTL 20140214)
[    0.000000] ACPI: SSDT 0x00000000FC00A570 000031 (v02 Xen    HVM 
00000000 INTL 20140214)
[    0.000000] Booting paravirtualized kernel on Xen HVM
[    0.000000] xen: PV spinlocks enabled
[    0.000000] xen:events: Using FIFO-based ABI
[    0.000000] xen:events: Xen HVM callback vector for event delivery is 
enabled
[    0.156221] clocksource: xen: mask: 0xffffffffffffffff max_cycles: 
0x1cd42e4dffb, max_idle_ns: 881590591483 ns
[    0.156244] Xen: using vcpuop timer interface
[    0.156253] installing Xen timer for CPU 0
[    0.157188] installing Xen timer for CPU 1
[    0.248050] xenbus: xs_reset_watches failed: -38
[    0.292506] xen: --> pirq=16 -> irq=9 (gsi=9)
[    0.464822] xen:balloon: Initialising balloon driver
[    0.468089] xen_balloon: Initialising balloon driver
[    0.476131] clocksource: Switched to clocksource xen
[    0.491289] xen: --> pirq=17 -> irq=8 (gsi=8)
[    0.491405] xen: --> pirq=18 -> irq=12 (gsi=12)
[    0.491511] xen: --> pirq=19 -> irq=1 (gsi=1)
[    0.491622] xen: --> pirq=20 -> irq=6 (gsi=6)
[    1.058087] xen: --> pirq=21 -> irq=24 (gsi=24)
[    1.058369] xen:grant_table: Grant tables using version 1 layout
[    1.091277] blkfront: xvda: flush diskcache: enabled; persistent 
grants: enabled; indirect descriptors: enabled;
[    1.100218] xen_netfront: Initialising Xen virtual ethernet driver
[    1.173298] xenbus_probe_frontend: Device with no driver: device/vkbd/0
[    2.692397] systemd[1]: Detected virtualization xen.
[    3.453534] input: Xen Virtual Keyboard as /devices/virtual/input/input5
[    3.454923] input: Xen Virtual Pointer as /devices/virtual/input/input6

Current linux kernels contains PV drivers, as I understand it. And based 
on the referenced
document, the above messages would seem to imply that this is a PVHv2 
guest here. At least
according to what the referenced document explains as how to identify a 
PVH guest. But
shouldn't this be an HVM guest, per the example config file?

I get that the wiki is stale, so I gotta ask questions:

How do I identify/characterize a a PVHv2/HVMlite guest on Xen 4.9?

What, precisely, -defines- one of these (PVHv2) guests?

Re: my prior question on documentation, how does the
current tech preview define one of these hybrid guests? what are the salient
aspects of said guests, and what is that we want to do to create one?

My apologies if this is a simplistic question, but some clarification 
would be
greatly appreciated.

Gary

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