Re: HVM/PVH Balloon crash

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Wed, Sep 29, 2021 at 03:32:15PM +0200, Jan Beulich wrote:
> On 27.09.2021 00:53, Elliott Mitchell wrote:
> > (XEN) Xen call trace:
> > (XEN)    [<ffff82d0402e8be0>] R 
> > arch/x86/mm/p2m.c#p2m_flush_table+0x240/0x260
> > (XEN)    [<ffff82d0402ec51c>] S p2m_flush_nestedp2m+0x1c/0x30
> > (XEN)    [<ffff82d0402e0528>] S 
> > arch/x86/mm/hap/hap.c#hap_write_p2m_entry+0x378/0x490
> 
> hap_write_p2m_entry() calling p2m_flush_nestedp2m() suggests that
> nestedhvm_enabled() was true for the domain. While we will want to
> fix this, nested virt is experimental (even in current staging),
> and hence there at least is no security concern.

Copy and paste from the xl.cfg man page:

       nestedhvm=BOOLEAN
           Enable or disables guest access to hardware virtualisation
           features, e.g. it allows a guest Operating System to also function
           as a hypervisor. You may want this option if you want to run
           another hypervisor (including another copy of Xen) within a Xen
           guest or to support a guest Operating System which uses hardware
           virtualisation extensions (e.g. Windows XP compatibility mode on
           more modern Windows OS).  This option is disabled by default.

"This option is disabled by default." doesn't mean "this is an
experimental feature with no security support and is likely to crash the
hypervisor".

More notably this is fully enabled in default builds of Xen.  Contrast
this with the stance of the ARM side with regards to ACPI.


> Can you confirm that by leaving nested off you don't run into this
> (or a similar) issue?

Hypervisor doesn't panic.  `xl dmesg` does end up with:

(XEN) p2m_pod_demand_populate: Dom72 out of PoD memory! (tot=524304 
ents=28773031 dom72)
(XEN) domain_crash called from p2m-pod.c:1233

Which is problematic.  maxmem for this domain is set to allow for trading
memory around, so it is desireable for it to successfully load even when
its maximum isn't available.


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