[PATCH v5 13/14] KVM: arm64: Fold redundant exit code checks out of fixup_guest_exit()

[Marc Zyngier]

On 04/05/18 17:05, Dave Martin wrote:
> The entire tail of fixup_guest_exit() is contained in if statements
> of the form if (x && *exit_code == ARM_EXCEPTION_TRAP).  As a result,
> we can check just once and bail out of the function early, allowing
> the remaining if conditions to be simplified.
> 
> The only awkward case is where *exit_code is changed to
> ARM_EXCEPTION_EL1_SERROR in the case of an illegal GICv2 CPU
> interface access: in that case, the GICv3 trap handling code is
> skipped using a goto.  This avoids pointlessly evaluating the
> static branch check for the GICv3 case, even though we can't have
> vgic_v2_cpuif_trap and vgic_v3_cpuif_trap true simultaneously
> unless we have a GICv3 and GICv2 on the host: that sounds stupid,
> but I haven't satisfied myself that it can't happen.

Indeed, this cannot happen, unless we decided to trap access to the
memory-mapped interface of a GICv3 implementation. We don't do that.

But I guess the goto also serves a visual clue that the two cases are
mutually exclusives. Small nit below though:

> 
> No functional change.
> 
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> ---
>  arch/arm64/kvm/hyp/switch.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 39e9166..be09c52 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -385,11 +385,13 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
>  	 * same PC once the SError has been injected, and replay the
>  	 * trapping instruction.
>  	 */
> -	if (*exit_code == ARM_EXCEPTION_TRAP && !__populate_fault_info(vcpu))
> +	if (*exit_code != ARM_EXCEPTION_TRAP)
> +		goto exit;
> +
> +	if (!__populate_fault_info(vcpu))
>  		return true;
>  
> -	if (static_branch_unlikely(&vgic_v2_cpuif_trap) &&
> -	    *exit_code == ARM_EXCEPTION_TRAP) {
> +	if (static_branch_unlikely(&vgic_v2_cpuif_trap)) {
>  		bool valid;
>  
>  		valid = kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_DABT_LOW &&
> @@ -414,12 +416,12 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
>  				if (!__skip_instr(vcpu))
>  					*vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
>  				*exit_code = ARM_EXCEPTION_EL1_SERROR;
> +				goto exit;

This goto...

>  			}

... should be placed here. If this was a data abort, it cannot be a
system register trap, and the below conditions cannot possibly apply.

>  		}
>  	}
>  
>  	if (static_branch_unlikely(&vgic_v3_cpuif_trap) &&
> -	    *exit_code == ARM_EXCEPTION_TRAP &&
>  	    (kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_SYS64 ||
>  	     kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_CP15_32)) {
>  		int ret = __vgic_v3_perform_cpuif_access(vcpu);
> @@ -428,6 +430,7 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
>  			return true;
>  	}
>  
> +exit:
>  	/* Return to the host kernel and handle the exit */
>  	return false;
>  }
> 

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...