[PATCH] KVM: arm/arm64: VGIC MMIO: add missing irq_lock

[Christoffer Dall]

On Tue, Mar 06, 2018 at 09:21:06AM +0000, Andre Przywara wrote:
> Our irq_is_pending() helper function accesses multiple members of the
> vgic_irq struct, so we need to hold the lock when calling it.

For the record I don't think this is necessarily a completely valid
conclusion.  The fact that you access multiple members of a struct is a
good indication that it might be a good idea to hold a lock, but it's
not as simple as that.

I think the only thing that could happen here is that a caller
mistakenly evaluates line_level when it shouldn't, but that would only
happen when changing the configuration of an irq from level to edge,
while the line_level is high, expecting the line_level to go down, and
the pending state to be subsequently reported as false, but we only
support changing the configuration of an interrupt when it's disabled,
and as a result this can only affect reads of the PENDR registers.

> Add that requirement as a comment to the definition and take the lock
> around the call in vgic_mmio_read_pending(), where we were missing it
> before.
> 
> Signed-off-by: Andre Przywara <andre.przywara at arm.com>

Note, I'm fine with this change, but I don't agree with the rationale.
The rationale is to take the lock on every use for consistency and to
make the code easier to reason about, but it's possible that some future
analysis in the future would relax this requirement if essential for
performance.

Thanks,
-Christoffer

> ---
>  virt/kvm/arm/vgic/vgic-mmio.c | 3 +++
>  virt/kvm/arm/vgic/vgic.h      | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> index 83d82bd7dc4e..dbe99d635c80 100644
> --- a/virt/kvm/arm/vgic/vgic-mmio.c
> +++ b/virt/kvm/arm/vgic/vgic-mmio.c
> @@ -113,9 +113,12 @@ unsigned long vgic_mmio_read_pending(struct kvm_vcpu *vcpu,
>  	/* Loop over all IRQs affected by this read */
>  	for (i = 0; i < len * 8; i++) {
>  		struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
> +		unsigned long flags;
>  
> +		spin_lock_irqsave(&irq->irq_lock, flags);
>  		if (irq_is_pending(irq))
>  			value |= (1U << i);
> +		spin_unlock_irqrestore(&irq->irq_lock, flags);
>  
>  		vgic_put_irq(vcpu->kvm, irq);
>  	}
> diff --git a/virt/kvm/arm/vgic/vgic.h b/virt/kvm/arm/vgic/vgic.h
> index 12c37b89f7a3..5b11859a1a1e 100644
> --- a/virt/kvm/arm/vgic/vgic.h
> +++ b/virt/kvm/arm/vgic/vgic.h
> @@ -96,6 +96,7 @@
>  /* we only support 64 kB translation table page size */
>  #define KVM_ITS_L1E_ADDR_MASK		GENMASK_ULL(51, 16)
>  
> +/* Requires the irq_lock to be held by the caller. */
>  static inline bool irq_is_pending(struct vgic_irq *irq)
>  {
>  	if (irq->config == VGIC_CONFIG_EDGE)
> -- 
> 2.14.1
>