Re: [PATCH 04/12] KVM: s390: implement GISA IPM related primitives

[David Hildenbrand <david@redhat.com>]

On 19.01.2018 11:11, Heiko Carstens wrote:
> On Thu, Jan 18, 2018 at 09:45:03PM +0100, David Hildenbrand wrote:
>> Actually, the problem is there are no atomic byte-based operations on
>> s390x without Interlocked-Access-Facility 2.
>>
>> Even __sync_fetch_and_or(&gisa->ipm, value) falls back to a
>> Compare-And-Swap loop. And Compare-And-Swap also operates at least on 32bit.
>>
>> So I assume there isn't too much we can do about it. As storage
>> locations following the u8 are also written - but in an atomic matter,
>> it should in general not matter.
>>
>> But can we avoid starting the bitmap at the beginning of the gisa?
>>
>> What about something like this:
>>
>> +void kvm_s390_gisa_set_ipm_gisc(struct kvm_s390_gisa *gisa, u8 gisc)
>> +{
>> +       set_bit_inv(gisc, (unsigned long *) &gisa->ipm);
>> +}
> 
> set_bit_inv() may use a csg instruction which requires an 8 byte alignment
> of the operand. What you propose would crash immediately.
> 
> The code written by Michael is fine as-is.
> 

That's unfortunate... and still looks hacky to me :) But if it works ...

-- 

Thanks,

David / dhildenb