Re: HVM/PVH Balloon crash

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.09.2021 22:47, Elliott Mitchell wrote:
> On Mon, Sep 06, 2021 at 09:52:17AM +0200, Jan Beulich wrote:
>> 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.
> 
> I've got very little idea what is occurring as I'm expecting to be doing
> ARM debugging, not x86 debugging.
> 
> I was starting to wonder whether this was widespread or not.  As such I
> was reporting the factors which might be different in my environment.
> 
> The one which sticks out is the computer has an older AMD processor (you
> a 100% Intel shop?).

No, AMD is as relevant to us as is Intel.

>  The processor has the AMD NPT feature, but a very
> early/limited IOMMU (according to Linux "AMD IOMMUv2 functionality not
> available").
> 
> Xen 4.14 refused to load the Domain 0 kernel as PVH (not enough of an
> IOMMU).

That sounds odd at the first glance - PVH simply requires that there be
an (enabled) IOMMU. Hence the only thing I could imagine is that Xen
doesn't enable the IOMMU in the first place for some reason.

Jan