From 46b708ea875f14f5496109df053624199f3aae87 Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Thu, 23 Jul 2015 15:24:03 +0200 Subject: [PATCH] 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 Reviewed-by: Cornelia Huck --- 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; + } } -- 2.7.4