Re: [PATCH v1 07/14] s390x/s390-hypercall: introduce DIAG500 STORAGE_LIMIT

[Halil Pasic]

On Tue, 1 Oct 2024 11:15:02 +0200
Christian Borntraeger <borntraeger@linux.ibm.com> wrote:
[..]
> >> So 500+4 should probably not cause any harm apart from branch prediction
> >> going wrong the first 2 or 3 notifies.
> >>
> >> 502 will make kvm_s390_handle_diag larger.  
> > 
> > What do you mean by this last paragraph?
[..]

> gcc has logic for switch statements that decide about branch table or
> a chained compare+jump. I think due to spectre gcc now avoids indirect
> branches as much as possible but still a larger switch statement might
> kick the decision from inline compare/jump to a branch table.
> 
> I am not worried in this particular case this was more or less a
> "what could go wrong".

Hm, you did state that "502 will make kvm_s390_handle_diag larger". I
suppose now we agree that 502 would not make kvm_s390_handle_diag larger.
Right?

I understood that you prefer 500+4 over 502 because the latter would
make kvm_s390_handle_diag larger. Now that we have, I hope clarified,
that 502 would not make the switch larger, do you still prefer 500+4?

BTW your insights are very appreciated!

Regards,
Halil