Re: [Xen-devel] [PATCH v3 5/8] ARM: VGIC: factor out vgic_connect_hw_irq()

[Andre Przywara <andre.przywara@xxxxxxxxxx>]

Hi,

Yeah! Locking discussions! Have fun below ;-)

On 30/01/18 13:19, Julien Grall wrote:
> Hi Andre,
> 
> On 24/01/18 18:10, Andre Przywara wrote:
>> At the moment we happily access VGIC internal data structures like
>> the rank and struct pending_irq in gic.c, which should be VGIC agnostic.
>>
>> Factor out a new function vgic_connect_hw_irq(), which allows a virtual
>> IRQ to be connected to a hardware IRQ (using the hw bit in the LR).
>>
>> This removes said accesses to VGIC data structures and improves
>> abstraction.
> 
> You are modifying the locking of the 2 functions. But I don't see how
> this is safe. Can you explain it?

Are you worried about anything particular? I will explain my reasoning
below, but feel free to point me to the cause of your gripes.

>>
>> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>
>> Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> ---
>>   xen/arch/arm/gic-vgic.c    | 31 +++++++++++++++++++++++++++++++
>>   xen/arch/arm/gic.c         | 42
>> ++++++------------------------------------
>>   xen/include/asm-arm/vgic.h |  2 ++
>>   3 files changed, 39 insertions(+), 36 deletions(-)
>>
>> diff --git a/xen/arch/arm/gic-vgic.c b/xen/arch/arm/gic-vgic.c
>> index 0a2958c5db..d44e4dacd3 100644
>> --- a/xen/arch/arm/gic-vgic.c
>> +++ b/xen/arch/arm/gic-vgic.c
>> @@ -411,6 +411,37 @@ void gic_dump_vgic_info(struct vcpu *v)
>>           printk("Pending irq=%d\n", p->irq);
>>   }
>>   +int vgic_connect_hw_irq(struct domain *d, struct vcpu *v, unsigned
>> int virq,
>> +                        struct irq_desc *desc)
>> +{
>> +    unsigned long flags;
>> +    /* Use vcpu0 to retrieve the pending_irq struct. Given that we only
>> +     * route SPIs to guests, it doesn't make any difference. */
> 
> Can you fix the coding style of the comment?

Sure, I wasn't sure if the original style was on purpose.

>> +    struct vcpu *v_target = vgic_get_target_vcpu(d->vcpu[0], virq);
>> +    struct vgic_irq_rank *rank = vgic_rank_irq(v_target, virq);
>> +    struct pending_irq *p = irq_to_pending(v_target, virq);
>> +    int ret = 0;
>> +
>> +    /* We are taking to rank lock to prevent parallel connections. */
>> +    vgic_lock_rank(v_target, rank, flags);

So the rank lock here protects against assigning different (or the
same?) hardware IRQs to some particular virtual IRQ. The rank lock is
not strictly correct, but a not-too-bad replacement for the missing
per-IRQ lock. To this degree it protects reading and assigning p->desc
in struct pending_irq.

>> +
>> +    if ( desc )
>> +    {
>> +        /* The VIRQ should not be already enabled by the guest */
>> +        if ( !p->desc &&
>> +             !test_bit(GIC_IRQ_GUEST_ENABLED, &p->status) )
>> +            p->desc = desc;
>> +        else
>> +            ret = -EBUSY;
>> +    }
>> +    else
>> +        p->desc = NULL;
>> +
>> +    vgic_unlock_rank(v_target, rank, flags);
>> +
>> +    return ret;
>> +}
>> +
>>   /*
>>    * Local variables:
>>    * mode: C
>> diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
>> index 4cb74d449e..d46a6d54b3 100644
>> --- a/xen/arch/arm/gic.c
>> +++ b/xen/arch/arm/gic.c
>> @@ -128,27 +128,12 @@ void gic_route_irq_to_xen(struct irq_desc *desc,
>> unsigned int priority)
>>   int gic_route_irq_to_guest(struct domain *d, unsigned int virq,
>>                              struct irq_desc *desc, unsigned int
>> priority)
>>   {
>> -    unsigned long flags;
>> -    /* Use vcpu0 to retrieve the pending_irq struct. Given that we only
>> -     * route SPIs to guests, it doesn't make any difference. */
>> -    struct vcpu *v_target = vgic_get_target_vcpu(d->vcpu[0], virq);
>> -    struct vgic_irq_rank *rank = vgic_rank_irq(v_target, virq);
>> -    struct pending_irq *p = irq_to_pending(v_target, virq);
>> -    int res = -EBUSY;
>> -
>>       ASSERT(spin_is_locked(&desc->lock));
>>       /* Caller has already checked that the IRQ is an SPI */
>>       ASSERT(virq >= 32);
>>       ASSERT(virq < vgic_num_irqs(d));
>>       ASSERT(!is_lpi(virq));
>>   -    vgic_lock_rank(v_target, rank, flags);
>> -
>> -    if ( p->desc ||
>> -         /* The VIRQ should not be already enabled by the guest */
>> -         test_bit(GIC_IRQ_GUEST_ENABLED, &p->status) )
>> -        goto out;
>> -
>>       desc->handler = gic_hw_ops->gic_guest_irq_type;

Assigning desc->handler is protected by the desc lock, which we check is
held above. This is independent from the virtual IRQ locking. This is
only leaving aside the issue you correctly point out below ...

>>       set_bit(_IRQ_GUEST, &desc->status);
> 
> This looks wrong to me. You don't want to execute any of the code below
> if you are not able to route the pIRQ. For instance because the vIRQ has
> already a desc assigned.

Ah, good point. Indeed I didn't consider the failure path. Should be
easily fixed, though. Thanks for catching this.

>>   @@ -156,31 +141,19 @@ int gic_route_irq_to_guest(struct domain *d,
>> unsigned int virq,
>>           gic_set_irq_type(desc, desc->arch.type);
>>       gic_set_irq_priority(desc, priority);
>>   -    p->desc = desc;
>> -    res = 0;
>> -
>> -out:
>> -    vgic_unlock_rank(v_target, rank, flags);
>> -
>> -    return res;
>> +    return vgic_connect_hw_irq(d, NULL, virq, desc);
>>   }
>>     /* This function only works with SPIs for now */
>>   int gic_remove_irq_from_guest(struct domain *d, unsigned int virq,
>>                                 struct irq_desc *desc)
>>   {
>> -    struct vcpu *v_target = vgic_get_target_vcpu(d->vcpu[0], virq);
>> -    struct vgic_irq_rank *rank = vgic_rank_irq(v_target, virq);
>> -    struct pending_irq *p = irq_to_pending(v_target, virq);
>> -    unsigned long flags;
>> +    int ret;
>>         ASSERT(spin_is_locked(&desc->lock));
>>       ASSERT(test_bit(_IRQ_GUEST, &desc->status));
>> -    ASSERT(p->desc == desc);
> 
> You dropped this assert but I don't see any replacement in the new code?
> We really want to make sure the caller will not do something dumb here
> (like passing a different desc).

So the first thing here is that I can't have anything dereferencing
struct pending_irq here. Secondly the rank lock (protecting the p->
structure) is only taken below, so nothing prevents this from changing
between the ASSERT and the lock, AFAICS.
And to be honest, I don't really get the purpose of this ASSERT: the
desc pointer is taken from the pending_irq in the caller, but without
any locks. So if I am not mistaken, it could race with a
gic_route_irq_to_xen(), and that would lead to the ASSERT triggering,
just because of this race and not because of the code being broken
ultimately.
I *could* get the irq_desc by calling the new vgic_get_hw_irq_desc() -
again. Not sure if that is useful, though.
Another possibility would be to rethink this whole functionality:
The only caller (release_guest_irq() in irq.c) gets a virtual IRQ
number, then finds the associated irq_desc, only to lock it. Then it
passes both the virtual IRQ number and the irq_desc to this function,
where both are rechecked. The reason for this redundancy seems to be
some locking order (irq_desc first?), however I can't find any
documentation about this.
So I wonder if we could just pass on only the virtual IRQ number, and
let it up to this function here to safely retrieve the right irq_desc.

>>       ASSERT(!is_lpi(virq));
>>   -    vgic_lock_rank(v_target, rank, flags);

I couldn't find what this lock protects here, so early at least. Until
the actual "p->desc = NULL;" line below nothing needs to be protected by
this lock, it's all already covered by the desc lock.
We only need the lock to eventually atomically remove the connection
between the h/w and the virtual IRQ, which is done in
vgic_connect_hw_irq() now.

Cheers,
Andre.

>> -
>>       if ( d->is_dying )
>>       {
>>           desc->handler->shutdown(desc);
>> @@ -198,19 +171,16 @@ int gic_remove_irq_from_guest(struct domain *d,
>> unsigned int virq,
>>            */
>>           if ( test_bit(_IRQ_INPROGRESS, &desc->status) ||
>>                !test_bit(_IRQ_DISABLED, &desc->status) )
>> -        {
>> -            vgic_unlock_rank(v_target, rank, flags);
>>               return -EBUSY;
>> -        }
>>       }
>>   +    ret = vgic_connect_hw_irq(d, NULL, virq, NULL);
>> +    if ( ret )
>> +        return ret;
>> +
>>       clear_bit(_IRQ_GUEST, &desc->status);
>>       desc->handler = &no_irq_type;
>>   -    p->desc = NULL;
>> -
>> -    vgic_unlock_rank(v_target, rank, flags);
>> -
>>       return 0;
>>   }
>>   diff --git a/xen/include/asm-arm/vgic.h b/xen/include/asm-arm/vgic.h
>> index 22c8502c95..f4240df371 100644
>> --- a/xen/include/asm-arm/vgic.h
>> +++ b/xen/include/asm-arm/vgic.h
>> @@ -219,6 +219,8 @@ int vgic_v2_init(struct domain *d, int *mmio_count);
>>   int vgic_v3_init(struct domain *d, int *mmio_count);
>>     bool vgic_evtchn_irq_pending(struct vcpu *v);
>> +int vgic_connect_hw_irq(struct domain *d, struct vcpu *v, unsigned
>> int virq,
>> +                        struct irq_desc *desc);
>>     extern int domain_vgic_register(struct domain *d, int *mmio_count);
>>   extern int vcpu_vgic_free(struct vcpu *v);
>>
> 
> Cheers,
> 

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel