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Re: [Qemu-devel] [PATCH/RFC 3/3] s390x/ais: disable ais for compat machi
From: |
Christian Borntraeger |
Subject: |
Re: [Qemu-devel] [PATCH/RFC 3/3] s390x/ais: disable ais for compat machines |
Date: |
Tue, 26 Sep 2017 15:32:41 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 |
On 09/26/2017 03:00 PM, David Hildenbrand wrote:
>
> Wonder when we can finally let go of these hacks. The host cpu model is
> not migration safe, therefore such things are expected to not work. Just
> think about trying to migrate from a kernel without AIS support to a
> kernel with AIS support. It is broken.
>
> AIS is just one feature that actually tells you that you are currently
> doing something evil. Other CPU features you lose on the way simply
> don't result in an error, but still migration could break silently
> afterwards, when the guest assumes it has certain CPU features.
>
>
> The main problem is that we have machines <= 2.7 that had no CPU model
> support. For these machines, migration should work just fine, as
> s390_has_feat(S390_FEAT_ADAPTER_INT_SUPPRESSION)
> will always return false.
> However, the guest will be presented the AIS bit, as soon as the
> capability is enabled (as we then don't manually set the stfle bitmap
> from QEMU), which is bad.
>
> So my point would be: don't turn on these new facilities if the cpu
> model is not allowed (<=2.7), but don't add ais_allowed() compat
> handling for any newer machines. If they use the host model, they have
> to assume that migration can break.
>
> I think using cpu_model_allowed() would be just fine to be used instead
> of ais_allowed().
Let me thing about that. I will send 2 (with fs->ais_supported gone) and 3
and we can then decide if we only want to apply 2 or both.
Now: regarding your -cpu host comment:
I checked all the migration stuff now with -cpu z13 and it seems that
migration is now broken for qemu 2.10->qemu 2.11 when using the
s390-ccw-virtio-2.9
machine due to the PCI fencing from Conny.
qemu-system-s390x: Unknown savevm section or instance 'PCIBUS' 0
qemu-system-s390x: load of migration failed: Invalid argument
- Re: [Qemu-devel] [PATCH/RFC 2/3] s390x/ais: enable ais when migration is available, (continued)
[Qemu-devel] [PATCH/RFC 1/3] s390x/ais: disable ais facility as it is broken, Christian Borntraeger, 2017/09/22
[Qemu-devel] [PATCH/RFC 3/3] s390x/ais: disable ais for compat machines, Christian Borntraeger, 2017/09/22
Re: [Qemu-devel] [PATCH/RFC 0/3] ais fixups for 2.11, Christian Borntraeger, 2017/09/22