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@xxxxxxxxxx>
Reviewed-by: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
---
arch/s390/kvm/intercept.c | 42 +++++++++++++++++++++---------------------
1 file changed, 21 insertions(+), 21 deletions(-)
diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 7365e8a..b4a5aa1 100644
--- 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;
+ }
}
--
2.4.3
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
