Subject: [PATCH v4 07/13] ARM: KVM: VGIC virtual CPU interface management

From: [email-masked] (Marc Zyngier)
Date: Mon, 03 Dec 2012 14:11:03 +0000

On 03/12/12 13:23, Will Deacon wrote:
> Hi Marc,
> 
> I've managed to look at some more of the vgic code, so here is some more
> feedback. I've still not got to the end of the series, but there's light at
> the end of the tunnel...
> 
> On Sat, Nov 10, 2012 at 03:45:05PM +0000, Christoffer Dall wrote:
>> From: Marc Zyngier <[email-masked]>
>>
>> Add VGIC virtual CPU interface code, picking pending interrupts
>> from the distributor and stashing them in the VGIC control interface
>> list registers.
>>
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> Signed-off-by: Christoffer Dall <[email-masked]>
>> ---
>>  arch/arm/include/asm/kvm_vgic.h |   41 +++++++
>>  arch/arm/kvm/vgic.c             |  226 +++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 266 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_vgic.h b/arch/arm/include/asm/kvm_vgic.h
>> index 9e60b1d..7229324 100644
>> --- a/arch/arm/include/asm/kvm_vgic.h
>> +++ b/arch/arm/include/asm/kvm_vgic.h
>> @@ -193,8 +193,45 @@ struct vgic_dist {
>>  };
>>
>>  struct vgic_cpu {
>> +#ifdef CONFIG_KVM_ARM_VGIC
>> +       /* per IRQ to LR mapping */
>> +       u8              vgic_irq_lr_map[VGIC_NR_IRQS];
> 
> per IRQ?

Yes. We need to track which IRQ maps to which LR (so we can piggyback a
pending interrupt on an active one).

>> +
>> +       /* Pending interrupts on this VCPU */
>> +       DECLARE_BITMAP( pending, VGIC_NR_IRQS);
>> +
>> +       /* Bitmap of used/free list registers */
>> +       DECLARE_BITMAP( lr_used, 64);
>> +
>> +       /* Number of list registers on this CPU */
>> +       int             nr_lr;
>> +
>> +       /* CPU vif control registers for world switch */
>> +       u32             vgic_hcr;
>> +       u32             vgic_vmcr;
>> +       u32             vgic_misr;      /* Saved only */
>> +       u32             vgic_eisr[2];   /* Saved only */
>> +       u32             vgic_elrsr[2];  /* Saved only */
>> +       u32             vgic_apr;
>> +       u32             vgic_lr[64];    /* A15 has only 4... */
>> +#endif
>>  };
> 
> Looks like we should have a #define for the maximum number of list registers,
> so we keep vgic_lr and lr_user in sync.

Indeed.

>>
>> +#define VGIC_HCR_EN            (1 << 0)
>> +#define VGIC_HCR_UIE           (1 << 1)
>> +
>> +#define VGIC_LR_VIRTUALID      (0x3ff << 0)
>> +#define VGIC_LR_PHYSID_CPUID   (7 << 10)
>> +#define VGIC_LR_STATE          (3 << 28)
>> +#define VGIC_LR_PENDING_BIT    (1 << 28)
>> +#define VGIC_LR_ACTIVE_BIT     (1 << 29)
>> +#define VGIC_LR_EOI            (1 << 19)
>> +
>> +#define VGIC_MISR_EOI          (1 << 0)
>> +#define VGIC_MISR_U            (1 << 1)
>> +
>> +#define LR_EMPTY       0xff
>> +
> 
> Could stick these in asm/hardware/gic.h. I know they're not used by the gic
> driver, but they're the same piece of architecture so it's probably worth
> keeping in one place.

This is on my list of things to do once the GIC code is shared between
arm and arm64. Could do it earlier if that makes more sense.

> You'd probably also want a s/VGIC/GICH/

Sure.

>>  struct kvm;
>>  struct kvm_vcpu;
>>  struct kvm_run;
>> @@ -202,9 +239,13 @@ struct kvm_exit_mmio;
>>
>>  #ifdef CONFIG_KVM_ARM_VGIC
>>  int kvm_vgic_set_addr(struct kvm *kvm, unsigned long type, u64 addr);
>> +void kvm_vgic_sync_to_cpu(struct kvm_vcpu *vcpu);
>> +void kvm_vgic_sync_from_cpu(struct kvm_vcpu *vcpu);
>> +int kvm_vgic_vcpu_pending_irq(struct kvm_vcpu *vcpu);
>>  bool vgic_handle_mmio(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>                       struct kvm_exit_mmio *mmio);
>>
>> +#define irqchip_in_kernel(k)   (!!((k)->arch.vgic.vctrl_base))
>>  #else
>>  static inline int kvm_vgic_hyp_init(void)
>>  {
>> diff --git a/arch/arm/kvm/vgic.c b/arch/arm/kvm/vgic.c
>> index 82feee8..d7cdec5 100644
>> --- a/arch/arm/kvm/vgic.c
>> +++ b/arch/arm/kvm/vgic.c
>> @@ -587,7 +587,25 @@ static void vgic_dispatch_sgi(struct kvm_vcpu *vcpu, u32 reg)
>>
>>  static int compute_pending_for_cpu(struct kvm_vcpu *vcpu)
>>  {
>> -       return 0;
>> +       struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
>> +       unsigned long *pending, *enabled, *pend;
>> +       int vcpu_id;
>> +
>> +       vcpu_id = vcpu->vcpu_id;
>> +       pend = vcpu->arch.vgic_cpu.pending;
>> +
>> +       pending = vgic_bitmap_get_cpu_map(&dist->irq_state, vcpu_id);
>> +       enabled = vgic_bitmap_get_cpu_map(&dist->irq_enabled, vcpu_id);
>> +       bitmap_and(pend, pending, enabled, 32);
> 
> pend and pending! vcpu_pending and dist_pending?

A lot of that code has already been reworked. See:
https://lists.cs.columbia.edu/pipermail/kvmarm/2012-November/004138.html

>> +
>> +       pending = vgic_bitmap_get_shared_map(&dist->irq_state);
>> +       enabled = vgic_bitmap_get_shared_map(&dist->irq_enabled);
>> +       bitmap_and(pend + 1, pending, enabled, VGIC_NR_SHARED_IRQS);
>> +       bitmap_and(pend + 1, pend + 1,
>> +                  vgic_bitmap_get_shared_map(&dist->irq_spi_target[vcpu_id]),
>> +                  VGIC_NR_SHARED_IRQS);
>> +
>> +       return (find_first_bit(pend, VGIC_NR_IRQS) < VGIC_NR_IRQS);
>>  }
>>
>>  /*
>> @@ -613,6 +631,212 @@ static void vgic_update_state(struct kvm *kvm)
>>         }
>>  }
>>
>> +#define LR_PHYSID(lr)          (((lr) & VGIC_LR_PHYSID_CPUID) >> 10)
> 
> Is VGIC_LR_PHYSID_CPUID wide enough for this? The CPUID is only 3 bits, but
> the interrupt ID could be larger. Or do you not supported hardware interrupt
> forwarding? (in which case, LR_PHYSID is a misleading name).

Hardware interrupt forwarding is not supported. PHYSID is the name of
the actual field in the spec, hence the name of the macro. LR_CPUID?

>> +#define MK_LR_PEND(src, irq)   (VGIC_LR_PENDING_BIT | ((src) << 10) | (irq))
>> +/*
>> + * Queue an interrupt to a CPU virtual interface. Return true on success,
>> + * or false if it wasn't possible to queue it.
>> + */
>> +static bool vgic_queue_irq(struct kvm_vcpu *vcpu, u8 sgi_source_id, int irq)
>> +{
>> +       struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>> +       struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
>> +       int lr, is_level;
>> +
>> +       /* Sanitize the input... */
>> +       BUG_ON(sgi_source_id & ~7);
> 
> sgi_source_id > MAX_SGI_SOURCES (or whatever we end up having for the SGI
> and PPI limits).

OK.

>> +       BUG_ON(sgi_source_id && irq > 15);
> 
> irq > MAX_PPI_SOURCES

OK.

>> +       BUG_ON(irq >= VGIC_NR_IRQS);
>> +
>> +       kvm_debug("Queue IRQ%d\n", irq);
>> +
>> +       lr = vgic_cpu->vgic_irq_lr_map[irq];
>> +       is_level = !vgic_irq_is_edge(dist, irq);
>> +
>> +       /* Do we have an active interrupt for the same CPUID? */
>> +       if (lr != LR_EMPTY &&
>> +           (LR_PHYSID(vgic_cpu->vgic_lr[lr]) == sgi_source_id)) {
> 
> Ok, so this does return the source.
> 
>> +               kvm_debug("LR%d piggyback for IRQ%d %x\n", lr, irq, vgic_cpu->vgic_lr[lr]);
>> +               BUG_ON(!test_bit(lr, vgic_cpu->lr_used));
>> +               vgic_cpu->vgic_lr[lr] |= VGIC_LR_PENDING_BIT;
>> +               if (is_level)
>> +                       vgic_cpu->vgic_lr[lr] |= VGIC_LR_EOI;
>> +               return true;
>> +       }
>> +
>> +       /* Try to use another LR for this interrupt */
>> +       lr = find_first_bit((unsigned long *)vgic_cpu->vgic_elrsr,
>> +                              vgic_cpu->nr_lr);
>> +       if (lr >= vgic_cpu->nr_lr)
>> +               return false;
>> +
>> +       kvm_debug("LR%d allocated for IRQ%d %x\n", lr, irq, sgi_source_id);
>> +       vgic_cpu->vgic_lr[lr] = MK_LR_PEND(sgi_source_id, irq);
>> +       if (is_level)
>> +               vgic_cpu->vgic_lr[lr] |= VGIC_LR_EOI;
>> +
>> +       vgic_cpu->vgic_irq_lr_map[irq] = lr;
>> +       clear_bit(lr, (unsigned long *)vgic_cpu->vgic_elrsr);
>> +       set_bit(lr, vgic_cpu->lr_used);
>> +
>> +       return true;
>> +}
> 
> I can't help but feel that this could be made cleaner by moving the
> level-specific EOI handling out into a separate function.

Do you mean having two functions, one for edge and the other for level?
Seems overkill to me. I could move the "if (is_level) ..." to a common
spot though.

>> +
>> +/*
>> + * Fill the list registers with pending interrupts before running the
>> + * guest.
>> + */
>> +static void __kvm_vgic_sync_to_cpu(struct kvm_vcpu *vcpu)
>> +{
>> +       struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>> +       struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
>> +       unsigned long *pending;
>> +       int i, c, vcpu_id;
>> +       int overflow = 0;
>> +
>> +       vcpu_id = vcpu->vcpu_id;
>> +
>> +       /*
>> +        * We may not have any pending interrupt, or the interrupts
>> +        * may have been serviced from another vcpu. In all cases,
>> +        * move along.
>> +        */
>> +       if (!kvm_vgic_vcpu_pending_irq(vcpu)) {
>> +               pr_debug("CPU%d has no pending interrupt\n", vcpu_id);
>> +               goto epilog;
>> +       }
>> +
>> +       /* SGIs */
>> +       pending = vgic_bitmap_get_cpu_map(&dist->irq_state, vcpu_id);
>> +       for_each_set_bit(i, vgic_cpu->pending, 16) {
>> +               unsigned long sources;
>> +
>> +               sources = dist->irq_sgi_sources[vcpu_id][i];
>> +               for_each_set_bit(c, &sources, 8) {
>> +                       if (!vgic_queue_irq(vcpu, c, i)) {
>> +                               overflow = 1;
>> +                               continue;
>> +                       }
> 
> If there are multiple sources, why do you need to queue the interrupt
> multiple times? I would have thought it could be collapsed into one.

Because SGIs from different sources *are* different interrupts. In an
n-CPU system (with n > 2), you could have some message passing system
based on interrupts, and you'd need to know which CPU is pinging you.

>> +
>> +                       clear_bit(c, &sources);
>> +               }
>> +
>> +               if (!sources)
>> +                       clear_bit(i, pending);
> 
> What does this signify and how does it happen? An SGI without a source
> sounds pretty weird...

See the clear_bit() just above. Once all the sources for this SGI are
cleared, we can make the interrupt not pending anymore.

>> +
>> +               dist->irq_sgi_sources[vcpu_id][i] = sources;
>> +       }
>> +
>> +       /* PPIs */
>> +       for_each_set_bit_from(i, vgic_cpu->pending, 32) {
>> +               if (!vgic_queue_irq(vcpu, 0, i)) {
>> +                       overflow = 1;
>> +                       continue;
>> +               }
>> +
>> +               clear_bit(i, pending);
> 
> You could lose the `continue' and stick the clear_bit in an else clause
> (same for SGIs and SPIs).

Sure.

>> +       }
>> +
>> +
>> +       /* SPIs */
>> +       pending = vgic_bitmap_get_shared_map(&dist->irq_state);
>> +       for_each_set_bit_from(i, vgic_cpu->pending, VGIC_NR_IRQS) {
>> +               if (vgic_bitmap_get_irq_val(&dist->irq_active, 0, i))
>> +                       continue; /* level interrupt, already queued */
>> +
>> +               if (!vgic_queue_irq(vcpu, 0, i)) {
>> +                       overflow = 1;
>> +                       continue;
>> +               }
>> +
>> +               /* Immediate clear on edge, set active on level */
>> +               if (vgic_irq_is_edge(dist, i)) {
>> +                       clear_bit(i - 32, pending);
>> +                       clear_bit(i, vgic_cpu->pending);
>> +               } else {
>> +                       vgic_bitmap_set_irq_val(&dist->irq_active, 0, i, 1);
>> +               }
>> +       }
> 
> Hmm, more of this edge/level handling trying to use the same code and it
> not really working.

Hmmm. Let me think of a better way to do this without ending up
duplicating too much code (it is complicated enough that I don't want to
maintain two copies of it).

>> +
>> +epilog:
>> +       if (overflow)
>> +               vgic_cpu->vgic_hcr |= VGIC_HCR_UIE;
>> +       else {
>> +               vgic_cpu->vgic_hcr &= ~VGIC_HCR_UIE;
>> +               /*
>> +                * We're about to run this VCPU, and we've consumed
>> +                * everything the distributor had in store for
>> +                * us. Claim we don't have anything pending. We'll
>> +                * adjust that if needed while exiting.
>> +                */
>> +               clear_bit(vcpu_id, &dist->irq_pending_on_cpu);
>> +       }
>> +}
>> +
>> +/*
>> + * Sync back the VGIC state after a guest run. We do not really touch
>> + * the distributor here (the irq_pending_on_cpu bit is safe to set),
>> + * so there is no need for taking its lock.
>> + */
>> +static void __kvm_vgic_sync_from_cpu(struct kvm_vcpu *vcpu)
>> +{
>> +       struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>> +       struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
>> +       int lr, pending;
>> +
>> +       /* Clear mappings for empty LRs */
>> +       for_each_set_bit(lr, (unsigned long *)vgic_cpu->vgic_elrsr,
>> +                        vgic_cpu->nr_lr) {
>> +               int irq;
>> +
>> +               if (!test_and_clear_bit(lr, vgic_cpu->lr_used))
>> +                       continue;
>> +
>> +               irq = vgic_cpu->vgic_lr[lr] & VGIC_LR_VIRTUALID;
>> +
>> +               BUG_ON(irq >= VGIC_NR_IRQS);
>> +               vgic_cpu->vgic_irq_lr_map[irq] = LR_EMPTY;
>> +       }
>> +
>> +       /* Check if we still have something up our sleeve... */
>> +       pending = find_first_zero_bit((unsigned long *)vgic_cpu->vgic_elrsr,
>> +                                     vgic_cpu->nr_lr);
> 
> Does this rely on timeliness of maintenance interrupts with respect to
> EOIs in the guest? i.e. if a maintenance interrupt is delayed (I can't
> see anything in the spec stating that they're synchronous) and you end up
> taking one here, will you accidentally re-pend the interrupt?

I don't think so. ELRSR only indicates that the list register is empty.
If we find a zero bit there, we flag that this vcpu has at least one
pending interrupt (in its list registers). A delayed maintenance
interrupt may race with this by also setting this bit if an interrupt is
still in the active state after being EOIed, but that's not a problem
(we just set_bit twice). A race between clear and set would be
problematic though.

>> +       if (pending < vgic_cpu->nr_lr) {
>> +               set_bit(vcpu->vcpu_id, &dist->irq_pending_on_cpu);
>> +               smp_mb();
> 
> What's this barrier for?

It is strategically placed to entertain the reviewer. And it does its
job! I'll nuke it, now that you found it. ;-)

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