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Re: [Qemu-devel] [PATCH v3 6/7] s390x/kvm: handle AP instruction interce

From: Halil Pasic
Subject: Re: [Qemu-devel] [PATCH v3 6/7] s390x/kvm: handle AP instruction interception
Date: Thu, 5 Apr 2018 19:17:47 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 04/05/2018 06:38 PM, Tony Krowiak wrote:
>> Hard to really give good advice without access to the documentation, but:
>> - If we tell the guest that the feature is available, but it does not
>>    get any cards to use, returning an empty matrix makes the most sense
>>    to me.
>> - I would not tie starting the guest to the presence of a vfio-ap
>>    device. Having the feature available in theory but without any
>>    devices actually being usable by the guest does not really sound
>>    wrong (can we hotplug this later?)
> For this phase of development, it is my opinion that introducing AP 
> instruction
> interception handlers is superfluous for the following reasons:
> 1. Interception handling was introduced solely to ensure an operation 
> exception would
>    not be injected into the guest when CPU model feature for AP (i.e., ap=on)
>    is specified but a VFIO AP device (i.e., -device vfio-ap,sysfsdev=$path)
>    is not.
> 2. The implementation of guest dedicated crypto adapters uses AP instruction
>    interpretation to virtualize AP devices for a guest. As such, the NQAP,
>    DQAP and most variants of the PQAP instructions will not be
>    intercepted.
> 3. Hot plugging AP devices is not being supported for this phase of 
> development.
> It is my opinion that introducing these interception handlers at this time is
> unnecessary and risks opening a can of worms that would be
> better dealt with in a subsequent phase. For that reason and the reasons 
> stated
> above, I think the better option is to terminate starting the guest if the
> CPU model feature for AP is enabled but an AP device is not defined for the
> guest. This restriction, of course, will be removed when hot plug is 
> implemented
> in a subsequent development phase.

I second that! I agree that having ap instructions but not having the
possibility to actually do AP crypto is probably not what the user wants.
Preventing such a guest form starting (with a nice message) sounds reasonable
to me.

I agree with Connie, the approach 'hold the line' (until future hotplugs)
is the most reasonable thing to do *in the long run*. But I think it's better
to limit ourselves to the simplest case for now, I don't see any problems
with doing the hotplug support later.


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