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!