Re: [RFC PATCH 06/45] KVM: arm/arm64: vgic-new: Implement virtual IRQ injection

Christoffer Dall
2016-04-06T14:23:58Z

On Tue, Apr 05, 2016 at 06:28:55PM +0100, Andre Przywara wrote:
> Hi,
>
> On 29/03/16 22:16, Christoffer Dall wrote:
> > On Fri, Mar 25, 2016 at 02:04:29AM +0000, Andre Przywara wrote:
> >> From: Christoffer Dall <christoffer.dall@linaro.org>
> >>
> >> Provide a vgic_queue_irq() function which decides whether a given
> >> IRQ needs to be queued to a VCPU's ap_list.
> >> This should be called whenever an IRQ became pending or got enabled,
> >
> > becomes pending or enabled,
> >
> >> either as a result of userspace injection, from in-kernel emulated
> >> devices like the architected timer or from MMIO accesses to the
> >> distributor emulation.
> >> Also provides the necessary functions to allow userland to inject an
> >> IRQ to a guest.
> >
> > Since this is the first code that starts using our locking mechanism, we
> > add some (hopefully) clear documentation of our locking strategy and
> > requirements along with this patch.
> >
> >> [Andre: refactor out vgic_queue_irq()]
> >>
> >> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> >> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> >> ---
> >> include/kvm/vgic/vgic.h | 3 +
> >> virt/kvm/arm/vgic/vgic.c | 181 +++++++++++++++++++++++++++++++++++++++++++++++
> >> virt/kvm/arm/vgic/vgic.h | 1 +
> >> 3 files changed, 185 insertions(+)
> >>
> >> diff --git a/include/kvm/vgic/vgic.h b/include/kvm/vgic/vgic.h
> >> index 659f8b1..f32b284 100644
> >> --- a/include/kvm/vgic/vgic.h
> >> +++ b/include/kvm/vgic/vgic.h
> >> @@ -178,6 +178,9 @@ struct vgic_cpu {
> >> struct list_head ap_list_head;
> >> };
> >>
> >> +int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int intid,
> >> + bool level);
> >> +
> >> #define irqchip_in_kernel(k) (!!((k)->arch.vgic.in_kernel))
> >> #define vgic_initialized(k) (false)
> >> #define vgic_ready(k) ((k)->arch.vgic.ready)
> >> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> >> index 8e34916..a95aabc 100644
> >> --- a/virt/kvm/arm/vgic/vgic.c
> >> +++ b/virt/kvm/arm/vgic/vgic.c
> >> @@ -19,8 +19,25 @@
> >>
> >> #include "vgic.h"
> >>
> >> +#define CREATE_TRACE_POINTS
> >> +#include "../trace.h"
> >> +
> >> struct vgic_global kvm_vgic_global_state;
> >>
> >> +/*
> >> + * Locking order is always:
> >> + * vgic_cpu->ap_list_lock
> >> + * vgic_irq->irq_lock
> >> + *
> >> + * (that is, always take the ap_list_lock before the struct vgic_irq lock).
> >> + *
> >> + * When taking more than one ap_list_lock at the same time, always take the
> >> + * lowest numbered VCPU's ap_list_lock first, so:
> >> + * vcpuX->vcpu_id < vcpuY->vcpu_id:
> >> + * spin_lock(vcpuX->arch.vgic_cpu.ap_list_lock);
> >> + * spin_lock(vcpuY->arch.vgic_cpu.ap_list_lock);
> >> + */
> >> +
> >> struct vgic_irq *vgic_get_irq(struct kvm *kvm, struct kvm_vcpu *vcpu,
> >> u32 intid)
> >> {
> >> @@ -39,3 +56,167 @@ struct vgic_irq *vgic_get_irq(struct kvm *kvm, struct kvm_vcpu *vcpu,
> >> WARN(1, "Looking up struct vgic_irq for reserved INTID");
> >> return NULL;
> >> }
> >> +
> >> +/**
> >> + * kvm_vgic_target_oracle - compute the target vcpu for an irq
> >> + *
> >> + * @irq: The irq to route. Must be already locked.
>
> ^^^^^^^^^^^^^^^^^^^^^^
>
> >> + *
> >> + * Based on the current state of the interrupt (enabled, pending,
> >> + * active, vcpu and target_vcpu), compute the next vcpu this should be
> >> + * given to. Return NULL if this shouldn't be injected at all.
> >> + */
> >> +static struct kvm_vcpu *vgic_target_oracle(struct vgic_irq *irq)
> >> +{
> >> + /* If the interrupt is active, it must stay on the current vcpu */
> >> + if (irq->active)
> >> + return irq->vcpu;
> >
> > we are not taking a lock here. What are the locking expectations? If
> > the expectarions are that the IRQ is locked when calling this function,
> > can we have a BIG FAT COMMENT saying that then?
>
> Do you mean really BIG FAT or is the above sufficient? (I guess not).
> I will make it more prominent.

well, maybe it doesn't have to be BIG FAT. But I did miss the existing
comment. I think it would be preferred to have a separate paragraph
explaining the locking expectaions, but perhaps I'm just
being stupid.

>
> > It seems to me that we are somehow expecting irq->active and irq->vcpu
> > to be in sync, but that's not necessarily the case if the IRQ is not
> > locked.
> >
> >> +
> >> + /* If enabled and pending, it can migrate to a new one */
> >
> > I think this comment should be rewritten to:
> >
> > If the IRQ is not active but enabled and pending, we should direct it to
> > its configured target VCPU.
> >
> >> + if (irq->enabled && irq->pending)
> >> + return irq->target_vcpu;
> >> +
> >> + /* Otherwise, it is considered idle */
> >
> > not sure what idle means here, I suggest something like:
> >
> > If neither active nor pending and enabled, then this IRQ should not be
> > queued to any VCPU.
> >
> >> + return NULL;
> >> +}
> >> +
> >> +/*
> >> + * Only valid injection if changing level for level-triggered IRQs or for a
> >> + * rising edge.
> >> + */
> >> +static bool vgic_validate_injection(struct vgic_irq *irq, bool level)
> >> +{
> >> + switch (irq->config) {
> >> + case VGIC_CONFIG_LEVEL:
> >> + return irq->line_level != level;
> >> + case VGIC_CONFIG_EDGE:
> >> + return level;
> >> + default:
> >> + BUG();
> >
> > is the default case there for making the compiler happy or can we just
> > get rid of it?
>
> Just removing it was fine (for GCC 5.3.0, at least).
>
> >> + }
> >> +}
> >> +
> >> +/*
> >> + * Check whether an IRQ needs to (and can) be queued to a VCPU's ap list.
> >> + * Do the queuing if necessary, taking the right locks in the right order.
> >> + * Returns true when the IRQ was queued, false otherwise.
> >> + *
> >> + * Needs to be entered with the IRQ lock already held, but will return
> >> + * with all locks dropped.
> >> + */
> >> +bool vgic_queue_irq(struct kvm *kvm, struct vgic_irq *irq)
> >
> > should we name this vgic_try_queue_irq_locked ?
>
> Mmh, since it (re-)tries quite hard I am not sure _try_ would be
> misleading. Basically it queues the IRQ whenever possible and/or
> sensible. Having _unlock in it like you suggested in another reply makes
> more sense, I think.

agreed

>
> >> +{
> >> + struct kvm_vcpu *vcpu = vgic_target_oracle(irq);
> >
> > should we have something like BUG_ON(!spin_is_locked(irq->irq_lock));
> > here?
> >
> > Not sure if there's some bug checking here which is only emitted if a
> > user select CONFIG_CHECK_SOME_LOCKING_THINGS that we could use...?
>
> There is CONFIG_DEBUG_SPINLOCK, but I couldn't find some conditional
> debug macro suitable for the purpose. I defined one now for the file
> only (since we have quite some users here).
>
> >> +
> >> + if (irq->vcpu || !(irq->pending && irq->enabled) || !vcpu) {
> >> + /*
> >> + * If this IRQ is already on a VCPU's ap_list, then it
> >> + * cannot be moved or modified and there is no more work for
> >> + * us to do.
> >> + *
> >> + * Otherwise, if the irq is not pending and enabled, it does
> >> + * not need to be inserted into an ap_list and there is also
> >> + * no more work for us to do.
> >> + */
> >
> > is the !vcpu check here not redundant because if you ever get to
> > evaluating it, then irq->vcpu is null, and pending and enabled are set,
> > which means the oracle couldn't have returned null, could it?
>
> In this case vcpu is always irq->target_vcpu, if I did the math
> correctly. So can this be NULL?
> Even if this is correct reasoning, I wonder if we optimize something
> prematurely here and rely on the current implementation of
> vgic_target_oracle(). I think the check for "!vcpu" is here to avoid a
> NULL pointer deference below (in the first spin_lock after the retry:
> label), so I'd rather keep this explicit check in here.

I'm really not a fan of building the correctness of one of the most
crucial parts of our code based on "let's add a few extra checks which
may not be necessary, just in case" kind of logic.

So let's be clear on why we have an if-statement here exactly:

As the comment says, if we can't move the IRQ, because it's already
assigned to somebody or if this IRQ is not pending or active, then it's
shouldn't be queued.

So the simple and all-encompassing check here is simply:

if (irq->vcpu || !vcpu) {
spin_unlock(&irq->irq_lock);
return false;
}

The only requirement for this to be correct is that the MMIO handler for
ISACTIVER to both set the active bit and the irq->vcpu pointer (and put
it on the AP list), without calling this function...). That was my
quesiton below.

Because if that's not the case, you could end up here with irq->active
set, but irq->vcpu == NULL and !(pending && enabled) and you'd error
out, which means you would have to check explicitly for the active state
here as well, but I think that just becomes too messy.

So, just change this to what I propose and we can deal with the active
state MMIO handler separately.

>
> > that would also explain why we don't have to re-check the same
> > conditions below...
> >
> > or am I getting this wrong, because you could also have someone
> > explicitly setting the IRQ to active via trapped MMIO, in which case we
> > should be able to queue it without it being pending && enabled, which
> > would indicate that it's the other way around, you should only evaluate
> > !vcpu and kup the !(pending && enabled) part....?
>
> You lost me here, which hints at the fragility of this optimization ;-)
>
> >> + spin_unlock(&irq->irq_lock);
> >> + return false;
> >> + }
> >> +
> >> + /*
> >> + * We must unlock the irq lock to take the ap_list_lock where
> >> + * we are going to insert this new pending interrupt.
> >> + */
> >> + spin_unlock(&irq->irq_lock);
> >> +
> >> + /* someone can do stuff here, which we re-check below */
> >> +retry:
> >> + spin_lock(&vcpu->arch.vgic_cpu.ap_list_lock);
> >> + spin_lock(&irq->irq_lock);
> >> +
> >> + /*
> >> + * Did something change behind our backs?
> >> + *
> >> + * There are two cases:
> >> + * 1) The irq became pending or active behind our backs and/or
> >> + * the irq->vcpu field was set correspondingly when putting
> >> + * the irq on an ap_list. Then drop the locks and return.
> >> + * 2) Someone changed the affinity on this irq behind our
> >> + * backs and we are now holding the wrong ap_list_lock.
> >> + * Then drop the locks and try the new VCPU.
> >> + */
> >> + if (irq->vcpu || !(irq->pending && irq->enabled)) {
> >
> > here I'm concerned about the active state again.
>
> Mmmh, can you elaborate and sketch a case where the active state would
> cause trouble? This check is just here to avoid iterating on a no longer
> pending or enabled IRQ. I wonder if an active IRQ can really sneak into
> this function here in the first place?

After having gone through the series I think we should deal with
the active state queing directly in the vgic_mmio_write_sactive()
function.

But I still prefer to move the retry label to the very top of this
function, and simplify these two statemtns to the condition I suggested:

if (unlinkely(irq->vcpu || vcpu != vgic_target_oracle(irq)))
goto retry;

The cost is that we perform a few additional checks at runtime in the
case where the IRQ was migrated while we released a lock (rare), but I
think it simplifies the code.

>
> > I feel like something more similar to my initial version of this patch
> > is what we really want:
> >
> > if (irq->vcpu || vcpu != vgic_target_oracle(irq))
> > goto real_retry;
> >
> > and read_retry is then a label at the very top of this function, before
> > the initial call to vgic_target_oracle()....
> >
> >> + spin_unlock(&irq->irq_lock);
> >> + spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);
> >> + return false;
> >> + }
> >> +
> >> + if (irq->target_vcpu != vcpu) {
> >> + spin_unlock(&irq->irq_lock);
> >> + spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);
> >> +
> >> + vcpu = irq->target_vcpu;
> >> + goto retry;
> >> + }
> >> +
> >> + list_add_tail(&irq->ap_list, &vcpu->arch.vgic_cpu.ap_list_head);
> >> + irq->vcpu = vcpu;
> >> +
> >> + spin_unlock(&irq->irq_lock);
> >> + spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);
> >> +
> >> + kvm_vcpu_kick(vcpu);
> >> +
> >> + return true;
> >> +}
> >> +
> >> +static void vgic_update_irq_pending(struct kvm *kvm, struct kvm_vcpu *vcpu,
> >> + u32 intid, bool level)
> >> +{
> >> + struct vgic_irq *irq = vgic_get_irq(kvm, vcpu, intid);
> >> +
> >> + trace_vgic_update_irq_pending(vcpu->vcpu_id, intid, level);
> >> +
> >> + BUG_ON(in_interrupt());
> >
> > I don't remember why we thought it was a good idea to have this BUG_ON()
> > anymore. Anyone?
>
> Me neither. Is that because of the case where "kvm_notify_acked_irq
> calls kvm_set_irq" (which in turn may call this function)?
> I am happy to remove it, also as the old VGIC doesn't seem to have it.

ok, nuke it.

>
> >> +
> >> + spin_lock(&irq->irq_lock);
> >> +
> >> + if (!vgic_validate_injection(irq, level)) {
> >> + /* Nothing to see here, move along... */
> >> + spin_unlock(&irq->irq_lock);
> >> + return;
> >> + }
> >> +
> >> + if (irq->config == VGIC_CONFIG_LEVEL) {
> >> + irq->line_level = level;
> >> + irq->pending = level || irq->soft_pending;
> >> + } else {
> >> + irq->pending = true;
> >> + }
> >> +
> >> + vgic_queue_irq(kvm, irq);
> >> +}
> >> +
> >> +/**
> >> + * kvm_vgic_inject_irq - Inject an IRQ from a device to the vgic
> >> + * @kvm: The VM structure pointer
> >> + * @cpuid: The CPU for PPIs
> >> + * @intid: The INTID to inject a new state to.
> >> + * must not be mapped to a HW interrupt.
> >
> > stray line here? I don't understand this bit about 'must not be mapped'
> > and I think that should be moved to the explanation below with some
> > rationale, and if important, perhaps guarded with a BUG_ON() ?
>
> I think this is a copy&paste leftover from the old VGIC with the old way
> of handling mapped IRQs. Actually the implementations of
> kvm_vgic_inject_irq() and kvm_vgic_inject_mapped_irq() are now
> identical, so the former differentiation does not apply anymore. I will
> #define the latter to the former for the new VGIC and we should schedule
> the removal of the the "mapped" version when the old VGIC gets removed.

sounds good.

>
> Btw: Are we OK with marking those cases which deserve some rework after
> the old VGIC is gone with some kind of TODO comments?
>

I really think we should avoid merging TODOs as much as possible, but in
this case it's an exported interface function which could be hard to
work around with the current vgic, so it may be an exception to the
rule.

Thanks,
-Christoffer