Re: [PATCH v4 6/9] KVM: arm64: vgic: Implement SW-driven deactivation

Alexandru Elisei
2021-06-23T14:14:20Z

Hi Marc,

On 6/22/21 5:12 PM, Marc Zyngier wrote:
> On Thu, 17 Jun 2021 15:58:31 +0100,
> Alexandru Elisei <alexandru.elisei@arm.com> wrote:
>> Hi Marc,
>>
>> On 6/1/21 11:40 AM, Marc Zyngier wrote:
>>> In order to deal with these systems that do not offer HW-based
>>> deactivation of interrupts, let implement a SW-based approach:
>> Nitpick, but shouldn't that be "let's"?
> "Let it be...". ;-) Yup.
>
>>> - When the irq is queued into a LR, treat it as a pure virtual
>>> interrupt and set the EOI flag in the LR.
>>>
>>> - When the interrupt state is read back from the LR, force a
>>> deactivation when the state is invalid (neither active nor
>>> pending)
>>>
>>> Interrupts requiring such treatment get the VGIC_SW_RESAMPLE flag.
>>>
>>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>>> ---
>>> arch/arm64/kvm/vgic/vgic-v2.c | 19 +++++++++++++++----
>>> arch/arm64/kvm/vgic/vgic-v3.c | 19 +++++++++++++++----
>>> include/kvm/arm_vgic.h | 10 ++++++++++
>>> 3 files changed, 40 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/arch/arm64/kvm/vgic/vgic-v2.c b/arch/arm64/kvm/vgic/vgic-v2.c
>>> index 11934c2af2f4..2c580204f1dc 100644
>>> --- a/arch/arm64/kvm/vgic/vgic-v2.c
>>> +++ b/arch/arm64/kvm/vgic/vgic-v2.c
>>> @@ -108,11 +108,22 @@ void vgic_v2_fold_lr_state(struct kvm_vcpu *vcpu)
>>> * If this causes us to lower the level, we have to also clear
>>> * the physical active state, since we will otherwise never be
>>> * told when the interrupt becomes asserted again.
>>> + *
>>> + * Another case is when the interrupt requires a helping hand
>>> + * on deactivation (no HW deactivation, for example).
>>> */
>>> - if (vgic_irq_is_mapped_level(irq) && (val & GICH_LR_PENDING_BIT)) {
>>> - irq->line_level = vgic_get_phys_line_level(irq);
>>> + if (vgic_irq_is_mapped_level(irq)) {
>>> + bool resample = false;
>>> +
>>> + if (val & GICH_LR_PENDING_BIT) {
>>> + irq->line_level = vgic_get_phys_line_level(irq);
>>> + resample = !irq->line_level;
>>> + } else if (vgic_irq_needs_resampling(irq) &&
>>> + !(irq->active || irq->pending_latch)) {
>> I'm having a hard time figuring out when and why a level sensitive
>> can have pending_latch = true.
>>
>> I looked kvm_vgic_inject_irq(), and that function sets pending_latch
>> only for edge triggered interrupts (it sets line_level for level
>> sensitive ones). But irq_is_pending() looks at **both**
>> pending_latch and line_level for level sensitive interrupts.
> Yes, and that's what an implementation requires.
>
>> The only place that I've found that sets pending_latch regardless of
>> the interrupt type is in vgic_mmio_write_spending() (called on a
>> trapped write to GICD_ISENABLER).
> Are you sure? It really should be GICD_ISPENDR. I'll assume that this
> is what you mean below.

Yes, that's what I meant, sorry for the confusion.

>
>> vgic_v2_populate_lr() clears
>> pending_latch only for edge triggered interrupts, so that leaves
>> vgic_v2_fold_lr_state() as the only function pending_latch is
>> cleared for level sensitive interrupts, when the interrupt has been
>> handled by the guest. Are we doing all of this to emulate the fact
>> that level sensitive interrupts (either purely virtual or hw mapped)
>> made pending by a write to GICD_ISENABLER remain pending until they
>> are handled by the guest?
> Yes, or cleared by a write to GICD_ICPENDR. You really need to think
> of the input into the GIC as some sort of OR gate combining both the
> line level and the PEND register. With a latch for edge interrupts.
>
> Have a look at Figure 4-10 ("Logic of the pending status of a
> level-sensitive interrupt") in the GICv2 arch spec (ARM IHI 0048B.b)
> to see what I actually mean.
>
>> If that is the case, then I think this is what the code is doing:
>>
>> - There's no functional change when the irqchip has HW deactivation
>>
>> - For level sensitive, hw mapped interrupts made pending by a write
>> to GICD_ISENABLER and not yet handled by the guest (pending_latch ==
>> true) we don't clear the pending state of the interrupt.
>>
>> - For level sensitive, hw mapped interrupts we clear the pending
>> state in the GIC and the device will assert the interrupt again if
>> it's still pending at the device 1level. I have a question about
>> this. Why don't we sample the interrupt state by calling
>> vgic_get_phys_line_level()? Because that would be slower than the
>> alternative that you are proposing here?
> Yes. It is *much* faster to read the timer status register (for
> example) than going via an MMIO access to read the (re)distributor
> that will return the same value.

Thank you for the explanation, much appreciated. The patch looks to me like it's
doing the right thing.

Thanks,

Alex