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Re: [qemu-s390x] [PATCH v2 5/5] s390x/cpumodel: Set up CPU model for AP

From: Tony Krowiak
Subject: Re: [qemu-s390x] [PATCH v2 5/5] s390x/cpumodel: Set up CPU model for AP device support
Date: Fri, 2 Mar 2018 11:07:41 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0

On 03/01/2018 09:12 AM, Pierre Morel wrote:
On 28/02/2018 12:40, Cornelia Huck wrote:
On Wed, 28 Feb 2018 11:26:30 +0100
David Hildenbrand <address@hidden> wrote:

Then I request the following change in KVM:

If KVM_S390_VM_CPU_FEAT_AP is enabled in KVM, ECA.28 is _always_ set
(not just if an AP device is configured). This especially makes things a
lot easier when it comes to handling hotplugged CPUs and avoiding race
conditions when enabling these bits as mentioned in the KVM series.

KVM_S390_VM_CPU_FEAT_AP == AP instructions available for the guest
(don't throw an operation exception).

So this feature then really is guest ABI. The instructions are
available. If there is no device configured, bad luck.
Sounds sensible from my POV.

I have a concern with this proposition and with the original code:

1) ap=on is a guest ABI feature saying to the guest you can use AP instructions

2) How we provide AP instructions to the guest can be done in three different ways:
 - SIE Interpretation
 - interception with VFIO
 - interception with emulation

3) We implement this with a device in QEMU and a certain level kernel support.

It seems possible to set or not ECA.28 , based on the type of kernel device:
- SIE interpretation -> MATRIX KVM device -> ECA.28
- Interception with VFIO and virtualization -> no ECA.28
- interception with emulation -> no ECA.28

I understand the concern with the vCPU but I think we can handle it with an indirect variable

SIE interpretation Device + KVM_S390_VM_CPU_FEAT_AP -> set the variable ap_to_be_sie_interpreted=1
Then in vCPU initialization set ECA.28 based on this variable.

I think it let us more doors open, what is your opinion?
I've already implemented a proof of concept similar to what you suggest to verify whether it would. I wasn't completely sure of the flow of control between the KVM notification to the device driver and the vcpu setup. If the variable is set when the device driver is notified about KVM, it has to happen before vcpu setup for this to work. I was able to verify that with my proof of concept. This discussion really belongs in the KVM/kernel patches, so I am going to continue
the discussion of my proposal there.



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