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

[Heiko Carstens <heiko.carstens@de.ibm.com>]

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.