Re: HVM/PVH Ballon crash

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.09.2021 00:10, Elliott Mitchell wrote:
> I brought this up a while back, but it still appears to be present and
> the latest observations appear rather serious.
> 
> I'm unsure of the entire set of conditions for reproduction.
> 
> Domain 0 on this machine is PV (I think the BIOS enables the IOMMU, but
> this is an older AMD IOMMU).
> 
> This has been confirmed with Xen 4.11 and Xen 4.14.  This includes
> Debian's patches, but those are mostly backports or environment
> adjustments.
> 
> Domain 0 is presently using a 4.19 kernel.
> 
> The trigger is creating a HVM or PVH domain where memory does not equal
> maxmem.

I take it you refer to "[PATCH] x86/pod: Do not fragment PoD memory
allocations" submitted very early this year? There you said the issue
was with a guest's maxmem exceeding host memory size. Here you seem to
be talking of PoD in its normal form of use. Personally I uses this
all the time (unless enabling PCI pass-through for a guest, for being
incompatible). I've not observed any badness as severe as you've
described.

> New observations:
> 
> I discovered this occurs with PVH domains in addition to HVM ones.
> 
> I got PVH GRUB operational.  PVH GRUB appeared at to operate normally
> and not trigger the crash/panic.
> 
> The crash/panic occurred some number of seconds after the Linux kernel
> was loaded.
> 
> 
> Mitigation by not using ballooning with HVM/PVH is workable, but this is
> quite a large mine in the configuration.
> 
> I'm wondering if perhaps it is actually the Linux kernel in Domain 0
> which is panicing.
> 
> The crash/panic occurring AFTER the main kernel loads suggests some
> action by the user domain is doing is the actual trigger of the
> crash/panic.

All of this is pretty vague: If you don't even know what component it
is that crashes / panics, I don't suppose you have any logs. Yet what
do you expect us to do without any technical detail?

Jan