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

From: David Hildenbrand
Subject: Re: [PATCH v5 07/18] s390x: protvirt: Inhibit balloon when switching to protected mode
Date: Wed, 26 Feb 2020 16:15:48 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 26.02.20 16:13, Christian Borntraeger wrote:
> 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.

+1, this is a global "don't use ram_block_discard" trigger.


David / dhildenb

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