Re: [PATCH v6] KVM: arm64: Add early_param to control WFx trapping

[Oliver Upton <oliver.upton@linux.dev>]

On Thu, May 23, 2024 at 05:40:55PM +0000, Colton Lewis wrote:
> Add an early_params to control WFI and WFE trapping. This is to
> control the degree guests can wait for interrupts on their own without
> being trapped by KVM. Options for each param are trap and notrap. trap
> enables the trap. notrap disables the trap. Note that when enabled,
> traps are allowed but not guaranteed by the CPU architecture. Absent
> an explicitly set policy, default to current behavior: disabling the
> trap if only a single task is running and enabling otherwise.
> 
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
> v6:
>  * Rebase to v6.9.1

As in from the stable tree? Please base your patches on an -rc tag, and
especially one from this release cycle.

> +static bool kvm_vcpu_should_clear_twi(struct kvm_vcpu *vcpu)
> +{
> +	if (likely(kvm_wfi_trap_policy == KVM_WFX_NOTRAP_SINGLE_TASK))
> +		return single_task_running() &&
> +			(atomic_read(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count) ||
> +			 vcpu->kvm->arch.vgic.nassgireq);
> +
> +	return kvm_wfi_trap_policy == KVM_WFX_NOTRAP;
> +}

Generally, it is more readable to organize your code in such a way that
multiline statements are unnested as much as possible. So if you were to
invert the if condition it'd become a bit cleaner.

Here is what I plan on squashing into this patch,
kvm_vcpu_should_clear_twe() got the same treatment for the sake of
consistency.

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9cddd1096b0a..53e23528d2cf 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -557,20 +557,20 @@ static void vcpu_set_pauth_traps(struct kvm_vcpu *vcpu)
 
 static bool kvm_vcpu_should_clear_twi(struct kvm_vcpu *vcpu)
 {
-	if (likely(kvm_wfi_trap_policy == KVM_WFX_NOTRAP_SINGLE_TASK))
-		return single_task_running() &&
-			(atomic_read(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count) ||
-			 vcpu->kvm->arch.vgic.nassgireq);
+	if (unlikely(kvm_wfi_trap_policy != KVM_WFX_NOTRAP_SINGLE_TASK))
+		return kvm_wfi_trap_policy == KVM_WFX_NOTRAP;
 
-	return kvm_wfi_trap_policy == KVM_WFX_NOTRAP;
+	return single_task_running() &&
+	       (atomic_read(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count) ||
+		vcpu->kvm->arch.vgic.nassgireq);
 }
 
 static bool kvm_vcpu_should_clear_twe(struct kvm_vcpu *vcpu)
 {
-	if (likely(kvm_wfe_trap_policy == KVM_WFX_NOTRAP_SINGLE_TASK))
-		return single_task_running();
+	if (unlikely(kvm_wfe_trap_policy != KVM_WFX_NOTRAP_SINGLE_TASK))
+		return kvm_wfe_trap_policy == KVM_WFX_NOTRAP;
 
-	return kvm_wfe_trap_policy == KVM_WFX_NOTRAP;
+	return single_task_running();
 }
 
 void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

-- 
Thanks,
Oliver