linux-kernel - Re: [PATCH v3 3/3] KVM: s390: vsie: Make use of CRYCB FORMAT2 clear

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Date:   Thu, 23 Aug 2018 14:11:00 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>, pmorel@...ux.ibm.com
Cc:     linux-kernel@...r.kernel.org, cohuck@...hat.com,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        akrowiak@...ux.ibm.com, borntraeger@...ibm.com,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH v3 3/3] KVM: s390: vsie: Make use of CRYCB FORMAT2 clear

On 8/23/18 2:03 PM, David Hildenbrand wrote:
> On 23.08.2018 13:53, Janosch Frank wrote:
>> On 8/23/18 1:47 PM, Pierre Morel wrote:
>>> On 23/08/2018 13:33, Janosch Frank wrote:
>>>> On 8/23/18 1:21 PM, David Hildenbrand wrote:
>>>>> On 23.08.2018 13:05, Janosch Frank wrote:
>>>>>> On 8/23/18 12:25 PM, Pierre Morel wrote:
>>>>>>> The comment preceding the shadow_crycb function is
>>>>>>> misleading, we effectively accept FORMAT2 CRYCB in the
>>>>>>> guest.
>>>>>>
>>>>>> I beg to differ:
>>>>>>
>>>>>> if (!(crycbd_o & vcpu->arch.sie_block->crycbd & CRYCB_FORMAT1))
>>>>>> 		return 0;
>>>>>
>>>>> FORMAT2 includes bit FORMAT1 (backwards compatible)
>>>>
>>>> Right, this check is very misleading because of the constant, we
>>>> effectively test against Format 0 and Format 2.
>>>>
>>>> Can we make this clearer by explicitly ANDing 0x01 or adding a comment?
>>>
>>> yes, done, I modified the comment in front of the function.
>>
>> Which is not what I want, what I want is:
>>
>> /* CRYCB_FORMAT2 includes the bit for CRYCB_FORMAT1, so we allow both
>> formats here */
>> if (!(crycbd_o & vcpu->arch.sie_block->crycbd & CRYCB_FORMAT1))
>> 	return 0;
> 
> While it's not wrong, it is also not required. And it might soon be
> obsolete again (with APXA, as you said, there we always have to check).
> 
> But I'll leave that to you
> 

I have not checked the vfio-ap patches, Pierre just told me that it goes
away in a few weeks anyway, so let's leave it out.



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