Re: [PATCH] x86/pod: Do not fragment PoD memory allocations

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Wed, Jan 27, 2021 at 10:47:19AM +0100, Jan Beulich wrote:
> On 26.01.2021 18:51, Elliott Mitchell wrote:
> > Okay, this has been reliably reproducing for a while.  I had originally
> > thought it was a problem of HVM plus memory != maxmem, but the
> > non-immediate restart disagrees with that assessment.
> 
> I guess it's not really clear what you mean with this, but anyway:
> The important aspect here that I'm concerned about is what the
> manifestations of the issue are. I'm still hoping that you would
> provide such information, so we can then start thinking about how
> to solve these. If, of course, there is anything worse than the
> expected effects which use of PoD can have on the guest itself.

Manifestation is domain 0 and/or Xen panic a few seconds after the
domain.cfg file is loaded via `xl`.  Everything on the host is lost and
the host restarts.  Any VMs which were present are lost and need to
restart, similar to power loss without UPS.

Upon pressing return for `xl create domain.cfg` there is a short period
of apparently normal behavior in domain 0.  After this there is a short
period of very laggy behavior in domain 0.  Finally domain 0 goes
unresponsive and so far by the time I've gotten to the host's console it
has already started to reboot.

The periods of apparently normal and laggy behavior are perhaps 5-10
seconds each.

The configurations I've reproduced with have had maxmem substantially
larger than the total host memory (this is intended as a prototype of a
future larger VM).  The first recorded observation of this was with
Debian's build of Xen 4.8, though I recall running into it with Xen 4.4
too.

Part of the problem might also be attributeable to QEMU touching all
memory on start (thus causing PoD to try to populate *all* memory) or
OVMF.


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