Still struggling to understand Xen

[Biff Eros <bifferos@xxxxxxxxx>]

Xen seems to be different to most other forms of virtualisation in the
way it presents hardware to the guest.  For so-called HVM guests I
understand everything:

Hypervisor in conjunction with dom0 provides disk and network devices
on PCI busses that can be viewed, enumerated with standard
off-the-shelf Linux drivers and tools.  This is all good.  My
confusion kicks in when the subject of PV drivers comes up.

>From what I understand (not clearly documented anywhere that I could
find) the hypervisor/dom0 combination somehow switches mode in
response to something the DomU guest does.  What exactly?  Don't know.
But by the time you've booted using the HVM hardware it seems the door
is shut, and any attempt to load front-end drivers will then result in
'device not found' messages or whatever.  That is, assuming my kernel
is configured correctly.

So this is presumably why most guests 'connect' to the PV back-end in
the initrd.  I couldn't really understand if it's the loading of the
conventional SCSI driver, or the detection of a SCSI device, or the
opening of a conventional SCSI device to mount as root that shuts the
above 'door'.  Unfortunately there isn't much documentation about
kernel configurations for Xen and what documentation I found seemed to
be out of date.

It's also unclear to me if the back-end drivers in a typical dom0 that
you might get from for example XCP-ng, or XenServer, or even AWS can
somehow be incompatible with the latest and greatest domU Linux
kernels.  Is there some kind of interface versioning or are all
versions forward and backward compatible?

I've been through pretty much all drivers related to Xen, compiled
them into my kernel and selected /dev/xvda1 device on boot, but it's
still not working for me, the Xen 'hardware' is not being detected, so
would appreciate any guidance you can offer.

Regards,
Mark.