linux-kernel - Re: [RFC 19/19] s390/facilities: enable AP facilities needed by guest

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Date:   Tue, 5 Dec 2017 10:14:15 -0500
From:   Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
To:     Pierre Morel <pmorel@...ux.vnet.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Harald Freudenberger <freude@...ux.vnet.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>, freude@...ibm.com,
        mjrosato@...ux.vnet.ibm.com, pasic@...ux.vnet.ibm.com,
        Boris Fiuczynski <fiuczy@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, heiko.carstens@...ibm.com,
        kwankhede@...dia.com, bjsdjshi@...ux.vnet.ibm.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        alifm@...ux.vnet.ibm.com, qemu-s390x@...gnu.org,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com
Subject: Re: [RFC 19/19] s390/facilities: enable AP facilities needed by guest

On 12/05/2017 09:23 AM, Pierre Morel wrote:
> On 05/12/2017 15:04, Cornelia Huck wrote:
>> On Tue, 5 Dec 2017 08:52:57 +0100
>> Harald Freudenberger <freude@...ux.vnet.ibm.com> wrote:
>>
>>> On 12/02/2017 02:30 AM, Tony Krowiak wrote:
>>
>>>> I agree with your suggestion that defining a new CPU model feature 
>>>> is probably
>>>> the best way to resolve this issue. The question is, should we 
>>>> define a single
>>>> feature indicating whether AP instructions are installed and set 
>>>> features bits
>>>> for the guest based on whether or not they are set in the linux 
>>>> host, or should
>>>> we define additional CPU model features for turning features bits 
>>>> on and off?
>>>> I guess it boils down to what behavior is expected for the AP bus 
>>>> running on
>>>> the linux guest. Here is a rundown of the facilities bits 
>>>> associated with AP
>>>> and how they affect the behavior of the AP bus:
>>>>
>>>> * STFLE.12 indicates whether the AP query function is available. If 
>>>> this bit
>>>>    is not set, then the AP bus scan will only test domains 0-15. 
>>>> For example,
>>>>    if adapters 4, 5, and 6 and domains 12 and 71 (0x47) are 
>>>> installed, then AP
>>>>    queues 04.0047, 05.0047 and 06.0047 will not be made available.
>>> STFLE 12 is the indication for Query AP Configuration Information 
>>> (QCI) available.
>>>> * STFLE.15 indicates whether the AP facilities test function is 
>>>> available. If
>>>>    this bit is not set, then the CEX4, CEX5 and CEX6 device drivers 
>>>> discovered
>>>>    by the AP bus scan will not get bound to any AP device drivers. 
>>>> Since theI think
>>>> STFLE.12
>>>>    AP matrix model supports only CEX4 and greater, no devices will 
>>>> be bound
>>>>    to any driver for a guest.
>>> This T-Bit extension to the TAPQ subfunction is a must have. When 
>>> kvm only
>>> supports CEX4 and upper then this bit could also act as the 
>>> indicator for
>>> AP instructions available. Of course if you want to implement pure 
>>> virtual
>>> full simulated AP without any real AP hardware on the host this bit 
>>> can't
>>> be the indicator.
>>
>> It would probably make sense to group these two together. Or is there
>> any advantage in supporting only a part of it?
>>
>>>> * STFLE.65 indicates whether AP interrupts are available. If this 
>>>> bit is not
>>>>    set, then the AP bus will use polling instead of using interrupt 
>>>> handlers
>>>>    to process AP events.
>>
>> So, does this indicate "adapter interrupts for AP" only? If so, we
>> should keep this separate and only enable it when we have the gisa etc.
>> ready.
>>
>
> Yes, STFLE 65, it is for AP only.
>
> QCI, STFLE 12, is no present on older systems, in this case AP uses 
> TAPQ to retrieve information for each AP
>
> So for my point of view, it make sense to separate the three 
> facilities to enable migration on older syste
In the interest of keeping things simple, this makes sense.
>
>
> Pierre
>
>