Re: [PATCH 17/31] KVM: arm64: vgic-v3: Enable trapping of Group-1 system registers

[Marc Zyngier <marc.zyngier@arm.com>]

On 30/05/17 10:07, Auger Eric wrote:
> Hi Marc,
> 
> On 03/05/2017 12:45, Marc Zyngier wrote:
>> In order to be able to trap Group-1 GICv3 system registers, we need to
>> set ICH_HCR_EL2.TALL1 begore entering the guest. This is conditionnaly
> before, conditionally
>> done after having restored the guest's state, and cleared on exit.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  include/linux/irqchip/arm-gic-v3.h | 1 +
>>  virt/kvm/arm/hyp/vgic-v3-sr.c      | 7 +++++++
>>  virt/kvm/arm/vgic/vgic-v3.c        | 4 ++++
>>  3 files changed, 12 insertions(+)
>>
>> diff --git a/include/linux/irqchip/arm-gic-v3.h b/include/linux/irqchip/arm-gic-v3.h
>> index c56d9bc2c904..a1739843343e 100644
>> --- a/include/linux/irqchip/arm-gic-v3.h
>> +++ b/include/linux/irqchip/arm-gic-v3.h
>> @@ -403,6 +403,7 @@
>>  
>>  #define ICH_HCR_EN			(1 << 0)
>>  #define ICH_HCR_UIE			(1 << 1)
>> +#define ICH_HCR_TALL1			(1 << 12)
>>  #define ICH_HCR_EOIcount_SHIFT		27
>>  #define ICH_HCR_EOIcount_MASK		(0x1f << ICH_HCR_EOIcount_SHIFT)
>>  
>> diff --git a/virt/kvm/arm/hyp/vgic-v3-sr.c b/virt/kvm/arm/hyp/vgic-v3-sr.c
>> index a521e105ade1..a27671b1e9af 100644
>> --- a/virt/kvm/arm/hyp/vgic-v3-sr.c
>> +++ b/virt/kvm/arm/hyp/vgic-v3-sr.c
>> @@ -257,6 +257,9 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
>>  			cpu_if->vgic_ap1r[0] = __vgic_v3_read_ap1rn(0);
>>  		}
>>  	} else {
>> +		if (static_branch_unlikely(&vgic_v3_cpuif_trap))
>> +			write_gicreg(0, ICH_HCR_EL2);
> Not directly related to this patch but this is not obvious to me why we
> reset the ICH_HCR_EL2 only when used_lrs != 0.

That's because if there is no interrupt to inject, then we've never
enabled the virtual CPU interface, and we've only configured VMCR.

>> +
>>  		cpu_if->vgic_elrsr = 0xffff;
>>  		cpu_if->vgic_ap0r[0] = 0;
>>  		cpu_if->vgic_ap0r[1] = 0;
>> @@ -329,6 +332,10 @@ void __hyp_text __vgic_v3_restore_state(struct kvm_vcpu *vcpu)
>>  
>>  		for (i = 0; i < used_lrs; i++)
>>  			__gic_v3_set_lr(cpu_if->vgic_lr[i], i);
>> +	} else {
>> +		/* Always write ICH_HCR_EL2 to enable trapping */
> "always" is a bit weird as this is conditional

Ah, true. How about:
		/*
                 * If we don't have any interrupt to inject, but that
		 * trapping is enabled, write the ICH_HCR_EL2 config.
		 */

>> +		if (static_branch_unlikely(&vgic_v3_cpuif_trap))
>> +			write_gicreg(cpu_if->vgic_hcr, ICH_HCR_EL2);
> and same question here. Why don't we always restore the ICH_HCR_EL2.
> Assuming when exiting the guest the vCPU I/F was enabled and used_lrs=0,

That case doesn't exist. The only case where we enable the virtual cpuif
is when we have something to inject (hence used_lrs != 0).

> when restoring don't we leave the vCPU I/F disabled? I must miss
> something but I don't find who is re-enabling the vCPU I/F in that case?

See above. This case shouldn't exist, and is only introduced here for
the benefit of the trapping.

>>  	}
>>  
>>  	/*
>> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
>> index 063526443781..547b8374fb64 100644
>> --- a/virt/kvm/arm/vgic/vgic-v3.c
>> +++ b/virt/kvm/arm/vgic/vgic-v3.c
>> @@ -21,6 +21,8 @@
>>  
>>  #include "vgic.h"
>>  
>> +static bool group1_trap;
>> +
>>  void vgic_v3_set_underflow(struct kvm_vcpu *vcpu)
>>  {
>>  	struct vgic_v3_cpu_if *cpuif = &vcpu->arch.vgic_cpu.vgic_v3;
>> @@ -239,6 +241,8 @@ void vgic_v3_enable(struct kvm_vcpu *vcpu)
>>  
>>  	/* Get the show on the road... */
>>  	vgic_v3->vgic_hcr = ICH_HCR_EN;
>> +	if (group1_trap)
> I don't remember the rationale behind using the bool here and using
> static_branch_unlikely in the other cases.

That's just the initial config path, before setting the static key.

> 
> May be good to squash the next patch to understand how group1_trap is set.

Sure, I'll have a look at that.

Thanks,

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