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From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Wed, 17 Jan 2018 15:44:39 +0100
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Subject: Re: [Qemu-devel] [PATCH 2/3] s390x/kvm: Handle bpb feature
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To: David Hildenbrand <david@redhat.com>, Cornelia Huck <cohuck@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, qemu-s390x <qemu-s390x@nongnu.org>, Alexander Graf <agraf@suse.de>, Thomas Huth <thuth@redhat.com>, Richard Henderson <rth@twiddle.net>, Janosch Frank <frankja@linux.vnet.ibm.com>, Halil Pasic <pasic@linux.vnet.ibm.com>



On 01/17/2018 03:30 PM, David Hildenbrand wrote:
> On 17.01.2018 15:18, Christian Borntraeger wrote:
>> We need to handle the bpb control on reset and migration. Normally
>> stfle.82 is transparent (and the normal guest part works without
>> hypervisor activity). To prevent any issues we require full
>> host kernel support for this feature.
> 
> Actually it is not transparent because we need hypervisor support to get
> VSIE running... or what am I missing? (or were you talking about bit 81?)

When you pass along the bit as a transparent bit (just enable it if the host
has it) it will work in nested guests. Its only that after reset or vsie you
have a short period of time where you work in a stale state.
Anyway we are aware that bit82 should have been a non-transparent bit, consider
it a quirk in the architecture. Thats why I handle this feature not in patch 3.