[Pkg-xen-devel] Experiment with rebuilt 4.14.2+25-gb6a8c4f72d-2

[Elliott Mitchell]

On Wed, Sep 01, 2021 at 03:35:41PM -0700, Elliott Mitchell wrote:
> I've been wanting PVH for a while, but hadn't gotten it working from
> Debian packages.  Trying to boot a PVH guest using pvGRUB produces an
> error similar to what Colin Watson saw in #776450:
> 
> xc: error: panic: xc_dom_elfloader.c:64: xc_dom_guest_type: image not capable of booting inside a HVM container: Invalid kernel
> libxl: error: libxl_dom.c:578:libxl__build_dom: xc_dom_parse_image failed
> 
> This might be an issue of the Debian GRUB 2.02 packages not liking
> Xen 4.14, but reading #776450 I'm wondering whether setups like this were
> ever working.

Thanks to Pry Mar, this got solved.  That error message is rather poor at
hinting the GRUB executables for PV and PVH are different.  Trying to
load the PV executable in PVH mode will fail.  GRUB needs a wishlist bug
to get the PV and PVH versions merged, or else provide a good error
message for being loaded in the wrong mode.


With that, I finally got to try to play with PVH.

Except I immediately discovered the issue I'd mentioned with HVM is also
an issue with PVH mode.  If memory does not equal maxmem, the system
crashes.  Fun thing is this time I was able to observe a bit.

One thing I got to observe was the crash didn't occur until the Linux
kernel loaded.  While GRUB was in operation, things were fine.  My take
on this is the hypercalls used by GRUB are functioning, but some
hypercall needed by the Linux kernel is the one which is broken.

In turn this alerts me to two places I hadn't previously been thinking
about.  One is xenstored, the other is the domain 0 kernel.  Of these the
Linux kernel seems rather more likely.  The PVH domain trying to access a
device and causing the domain 0 kernel to panic seems a good fit for the
symptoms.

Given how often the Xen drivers are getting broken and how Xen Linux
patches aren't tending to get backported, this seems a good candidate.


I'm waiting for one outside thing to get updated, hopefully then I'll be
on the new stable.  Hopefully Xen/ARM will have support for another OS
soon too.


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