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Commit 46b708ea authored by Christian Borntraeger's avatar Christian Borntraeger
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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: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: default avatarCornelia Huck <cornelia.huck@de.ibm.com>
parent 58c383c6

arch/s390/kvm/intercept.c

+21 −21
Original line number Diff line number Diff line
@@ -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))

		return -EOPNOTSUPP;

	func = intercept_funcs[code >> 2];

	if (func)

		return func(vcpu);

	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;

	}

}