KVM: s390: use simple switch statement as multiplexer

We currently do some magic shifting (by exploiting that exit codes
are always a multiple of 4) and a table lookup to jump into the
exit handlers. This causes some calculations and checks, just to
do an potentially expensive function call.

Changing that to a switch statement gives the compiler the chance
to inline and dynamically decide between jump tables or inline
compare and branches. In addition it makes the code more readable.

bloat-o-meter gives me a small reduction in code size:

add/remove: 0/7 grow/shrink: 1/1 up/down: 986/-1334 (-348)
function                                     old     new   delta
kvm_handle_sie_intercept                      72    1058    +986
handle_prog                                  704     696      -8
handle_noop                                   54       -     -54
handle_partial_execution                      60       -     -60
intercept_funcs                              120       -    -120
handle_instruction                           198       -    -198
handle_validity                              210       -    -210
handle_stop                                  316       -    -316
handle_external_interrupt                    368       -    -368

Right now my gcc does conditional branches instead of jump tables.
The inlining seems to give us enough cycles as some micro-benchmarking
shows minimal improvements, but still in noise.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 7365e8a4603216828813d80f197cfb59c0865149..b4a5aa110cec0b7ea62a534769812a2782b8cc35 100644 (file)
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -336,28 +336,28 @@ static int handle_partial_execution(struct kvm_vcpu *vcpu)
        return -EOPNOTSUPP;
 }
 
-static const intercept_handler_t intercept_funcs[] = {
-       [0x00 >> 2] = handle_noop,
-       [0x04 >> 2] = handle_instruction,
-       [0x08 >> 2] = handle_prog,
-       [0x10 >> 2] = handle_noop,
-       [0x14 >> 2] = handle_external_interrupt,
-       [0x18 >> 2] = handle_noop,
-       [0x1C >> 2] = kvm_s390_handle_wait,
-       [0x20 >> 2] = handle_validity,
-       [0x28 >> 2] = handle_stop,
-       [0x38 >> 2] = handle_partial_execution,
-};
-
 int kvm_handle_sie_intercept(struct kvm_vcpu *vcpu)
 {
-       intercept_handler_t func;
-       u8 code = vcpu->arch.sie_block->icptcode;
-
-       if (code & 3 || (code >> 2) >= ARRAY_SIZE(intercept_funcs))
+       switch (vcpu->arch.sie_block->icptcode) {
+       case 0x00:
+       case 0x10:
+       case 0x18:
+               return handle_noop(vcpu);
+       case 0x04:
+               return handle_instruction(vcpu);
+       case 0x08:
+               return handle_prog(vcpu);
+       case 0x14:
+               return handle_external_interrupt(vcpu);
+       case 0x1c:
+               return kvm_s390_handle_wait(vcpu);
+       case 0x20:
+               return handle_validity(vcpu);
+       case 0x28:
+               return handle_stop(vcpu);
+       case 0x38:
+               return handle_partial_execution(vcpu);
+       default:
                return -EOPNOTSUPP;
-       func = intercept_funcs[code >> 2];
-       if (func)
-               return func(vcpu);
-       return -EOPNOTSUPP;
+       }
 }