linux-kernel - Re: [PATCH v3 53/59] KVM: arm/arm64: GICv4: Hook vPE scheduling into vgic flush/sync

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Message-ID: <4dcaea2f-f8b4-c12e-c3c1-b43a4e29279b@arm.com>
Date:   Wed, 30 Aug 2017 10:59:46 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Christoffer Dall <cdall@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Eric Auger <eric.auger@...hat.com>,
        Shanker Donthineni <shankerd@...eaurora.org>,
        Mark Rutland <mark.rutland@....com>,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH v3 53/59] KVM: arm/arm64: GICv4: Hook vPE scheduling into
 vgic flush/sync

On 28/08/17 19:17, Christoffer Dall wrote:
> On Mon, Jul 31, 2017 at 06:26:31PM +0100, Marc Zyngier wrote:
>> The redistributor needs to be told which vPE is about to be run,
>> and tells us whether there is any pending VLPI on exit.
>>
>> Let's add the scheduling calls to the vgic flush/sync functions,
>> allowing the VLPIs to be delivered to the guest.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
>> ---
>>  virt/kvm/arm/vgic/vgic-v4.c | 24 ++++++++++++++++++++++++
>>  virt/kvm/arm/vgic/vgic.c    |  4 ++++
>>  virt/kvm/arm/vgic/vgic.h    |  1 +
>>  3 files changed, 29 insertions(+)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c
>> index 50721c4e3da5..0a8deefbcf1c 100644
>> --- a/virt/kvm/arm/vgic/vgic-v4.c
>> +++ b/virt/kvm/arm/vgic/vgic-v4.c
>> @@ -119,6 +119,30 @@ void vgic_v4_teardown(struct kvm *kvm)
>>  	its_vm->vpes = NULL;
>>  }
>>  
>> +int vgic_v4_schedule(struct kvm_vcpu *vcpu, bool on)
>> +{
>> +	int irq = vcpu->arch.vgic_cpu.vgic_v3.its_vpe.irq;
>> +
>> +	if (!vgic_is_v4_capable(vcpu->kvm) || !irq)
>> +		return 0;
> 
> why do we need to check the its_vpe.irq here?  This check is certainly
> not untuitive, as I don't understand what happened on a v4 capable
> system that somehow failed.  Is it because a specific VM is configured
> to not use VLPIs, or?

Hmm. I think that's a debug leftover from my early attempt at making
things work with QEMU, which initializes things in the opposite order
as kvmtool. It should be removed (or replaced by a fat WARN_ON).

>> +
>> +	/*
>> +	 * Before making the VPE resident, make sure the redistributor
>> +	 * expects us here.
>> +	 */
>> +	if (on) {
>> +		int err;
>> +
>> +		err = irq_set_affinity(irq, cpumask_of(smp_processor_id()));
> 
> This is pretty unintuitive, and coming here without having read your
> documentation may make people completely puzzled.  Could we provide a
> pointer to the documentation that explains how the vpe irq hooks this
> all together?

Sure, will do.

> 
>> +		if (err) {
>> +			kvm_err("failed irq_set_affinity IRQ%d (%d)\n", irq, err);
>> +			return err;
>> +		}
>> +	}
>> +
>> +	return its_schedule_vpe(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe, on);
>> +}
>> +
> 
> I'd prefer this function be split into two and follow the vgic notation
> of having a flush and a sync function.

Yes, makes sense.

>>  static struct vgic_its *vgic_get_its(struct kvm *kvm,
>>  				     struct kvm_kernel_irq_routing_entry *irq_entry)
>>  {
>> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
>> index dfac894f6f03..9ab52108989d 100644
>> --- a/virt/kvm/arm/vgic/vgic.c
>> +++ b/virt/kvm/arm/vgic/vgic.c
>> @@ -721,6 +721,8 @@ void kvm_vgic_sync_hwstate(struct kvm_vcpu *vcpu)
>>  {
>>  	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>>  
>> +	WARN_ON(vgic_v4_schedule(vcpu, false));
>> +
> 
> This is in the critical path, so would it be worth considering a static
> key to cater for non-GICv4 systems here?

Hey, for once I wasn't trying to optimize early! ;-) This would be
useful indeed, as I expect GICv4 systems to be the absolute minority for
the foreseeable future.

> 
>>  	/* An empty ap_list_head implies used_lrs == 0 */
>>  	if (list_empty(&vcpu->arch.vgic_cpu.ap_list_head))
>>  		return;
>> @@ -733,6 +735,8 @@ void kvm_vgic_sync_hwstate(struct kvm_vcpu *vcpu)
>>  /* Flush our emulation state into the GIC hardware before entering the guest. */
>>  void kvm_vgic_flush_hwstate(struct kvm_vcpu *vcpu)
>>  {
>> +	WARN_ON(vgic_v4_schedule(vcpu, true));
>> +
>>  	/*
>>  	 * If there are no virtual interrupts active or pending for this
>>  	 * VCPU, then there is no work to do and we can bail out without
>> diff --git a/virt/kvm/arm/vgic/vgic.h b/virt/kvm/arm/vgic/vgic.h
>> index 1210bf4681dc..693b654acf4d 100644
>> --- a/virt/kvm/arm/vgic/vgic.h
>> +++ b/virt/kvm/arm/vgic/vgic.h
>> @@ -234,5 +234,6 @@ int update_lpi_config(struct kvm *kvm, struct vgic_irq *irq,
>>  bool vgic_is_v4_capable(struct kvm *kvm);
>>  int vgic_v4_init(struct kvm *kvm);
>>  void vgic_v4_teardown(struct kvm *kvm);
>> +int vgic_v4_schedule(struct kvm_vcpu *vcpu, bool on);
>>  
>>  #endif
>> -- 
>> 2.11.0
>>
> Functionally, this looks correct.
> 
> Thanks,
> -Christoffer
> 

Thanks,

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