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Re: [PATCH v5 07/18] s390x: protvirt: Inhibit balloon when switching to

From: Christian Borntraeger
Subject: Re: [PATCH v5 07/18] s390x: protvirt: Inhibit balloon when switching to protected mode
Date: Wed, 26 Feb 2020 16:13:43 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 26.02.20 16:11, Janosch Frank wrote:
> On 2/26/20 3:59 PM, David Hildenbrand wrote:
>> On 26.02.20 13:20, Janosch Frank wrote:
>>> Ballooning in protected VMs can only be done when the guest shares the
>>> pages it gives to the host. Hence, until we have a solution for this
>>> in the guest kernel, we inhibit ballooning when switching into
>>> protected mode and reverse that once we move out of it.
>> I don't understand what you mean here, sorry. zapping a page will mean
>> that a fresh one will be faulted in when accessed. And AFAIK, that means
>> it will be encrypted again when needed.
> Yes, as soon as the host alters non-shared memory we'll run into
> integrity issues.
> I've been talking to Halil after I sent this out and it looks like we'll
> rather try to automatically enable the IOMMU for all devices when
> switching into protected mode. He said that if the IOMMU is set the
> balloon code will do an early exit on feature negotiation.

I think we should fence the balloon here nevertheless, so the patch in 
itself is probably fine.

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