Re: [PATCH v4] KVM: arm64: Add early_param to control WFx trapping

[Colton Lewis]

Oliver Upton <oliver.upton@xxxxxxxxx> writes:

> Hi Colton,

> On Mon, Apr 22, 2024 at 06:17:16PM +0000, Colton Lewis wrote:
> > @@ -2653,6 +2653,27 @@
> >   			[KVM,ARM] Allow use of GICv4 for direct injection of
> >   			LPIs.

> > +	kvm-arm.wfe_trap_policy=
> > +			[KVM,ARM] Control when to set wfe instruction trap.

> nitpick: when referring to the instruction, please capitalize it.

> Also, it doesn't hurt to be verbose here and say this cmdline option
> "Controls the WFE instruction trap behavior for KVM VMs"

> I say this because there is a separate set of trap controls that allow
> WFE or WFI to execute in EL0 (i.e. host userspace).

Will do.

> > +			trap: set wfe instruction trap
> > +
> > +			notrap: clear wfe instruction trap
> > +
> > +			default: set wfe instruction trap only if multiple
> > +				 tasks are running on the CPU

> I would strongly prefer we not make any default behavior user-visible.
> The default KVM behavior can (and will) change in the future.

> Only the absence of an explicit trap / notrap policy should fall back to
> KVM's default heuristics.

Makes sense to me. Will do.

> > -static inline void vcpu_clear_wfx_traps(struct kvm_vcpu *vcpu)
> > +static inline void vcpu_clear_wfe_trap(struct kvm_vcpu *vcpu)
> >   {
> >   	vcpu->arch.hcr_el2 &= ~HCR_TWE;
> > +}
> > +
> > +static inline void vcpu_clear_wfi_trap(struct kvm_vcpu *vcpu)
> > +{
> >   	if (atomic_read(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count) ||
> >   	    vcpu->kvm->arch.vgic.nassgireq)
> >   		vcpu->arch.hcr_el2 &= ~HCR_TWI;
> > @@ -119,12 +123,28 @@ static inline void vcpu_clear_wfx_traps(struct  
> > kvm_vcpu *vcpu)
> >   		vcpu->arch.hcr_el2 |= HCR_TWI;
> >   }

> This helper definitely does not do as it says on the tin. It ignores the
> policy requested on the command line and conditionally *sets* TWI. If
> the operator believes they know best and ask for a particular trap policy
> KVM should uphold it unconditionally. Even if they've managed to shoot
> themselves in the foot.

Will do. I was only splitting up what the existing helper did here.

> > @@ -423,6 +425,12 @@ void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)

> >   }

> > +static bool kvm_should_clear_wfx_trap(enum kvm_wfx_trap_policy p)
> > +{
> > +	return (p == KVM_WFX_NOTRAP && kvm_vgic_global_state.has_gicv4)
> > +		|| (p == KVM_WFX_NOTRAP_SINGLE_TASK && single_task_running());
> > +}

> style nitpick: operators should always go on the preceding line for a
> multi-line statement.

Will do.

> >   void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >   {
> >   	struct kvm_s2_mmu *mmu;
> > @@ -456,10 +464,15 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int  
> > cpu)
> >   	if (kvm_arm_is_pvtime_enabled(&vcpu->arch))
> >   		kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);

> > -	if (single_task_running())
> > -		vcpu_clear_wfx_traps(vcpu);
> > +	if (kvm_should_clear_wfx_trap(kvm_wfi_trap_policy))
> > +		vcpu_clear_wfi_trap(vcpu);
> >   	else
> > -		vcpu_set_wfx_traps(vcpu);
> > +		vcpu_set_wfi_trap(vcpu);
> > +
> > +	if (kvm_should_clear_wfx_trap(kvm_wfe_trap_policy))
> > +		vcpu_clear_wfe_trap(vcpu);
> > +	else
> > +		vcpu_set_wfe_trap(vcpu);

> >   	if (vcpu_has_ptrauth(vcpu))
> >   		vcpu_ptrauth_disable(vcpu);

> I find all of the layering rather hard to follow; we don't need
> accessors for doing simple bit manipulation.

> Rough sketch:

> static bool kvm_vcpu_should_clear_twi(struct kvm_vcpu *vcpu)
> {
> 	if (unlikely(kvm_wfi_trap != KVM_WFX_DEFAULT))
> 		return kvm_wfi_trap == 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 (unlikely(kvm_wfe_trap != KVM_WFX_DEFAULT))
> 		return kvm_wfe_trap == KVM_WFX_NOTRAP;

> 	return single_task_running();
> }

> static void kvm_vcpu_load_compute_hcr(struct kvm_vcpu *vcpu)
> {
> 	vcpu->arch.hcr_el2 |= HCR_TWE | HCR_TWI;

> 	if (kvm_vcpu_should_clear_twe(vcpu))
> 		vcpu->arch.hcr_el2 &= ~HCR_TWE;
> 	if (kvm_vcpu_should_clear_twi(vcpu))
> 		vcpu->arch.hcr_el2 &= ~HCR_TWI;
> }

Will do.

> And if we really wanted to, the non-default trap configuration could be
> moved to vcpu_reset_hcr() if we cared.

Might as well.