Re: [PATCH 2/2] xen: privcmd: Add support for ioeventfd

["Alex Bennée" <alex.bennee@linaro.org>]

Viresh Kumar <viresh.kumar@linaro.org> writes:

> On 29-09-23, 07:46, Juergen Gross wrote:
>> On 29.08.23 14:29, Viresh Kumar wrote:
>> > +static irqreturn_t ioeventfd_interrupt(int irq, void *dev_id)
>> > +{
>> > +	struct ioreq_port *port = dev_id;
>> > +	struct privcmd_kernel_ioreq *kioreq = port->kioreq;
>> > +	struct ioreq *ioreq = &kioreq->ioreq[port->vcpu];
>> > +	struct privcmd_kernel_ioeventfd *kioeventfd;
>> > +	unsigned int state = STATE_IOREQ_READY;
>> > +
>> > +	if (ioreq->state != STATE_IOREQ_READY ||
>> > +	    ioreq->type != IOREQ_TYPE_COPY || ioreq->dir != IOREQ_WRITE)
>> > +		return IRQ_NONE;
>> > +
>> > +	smp_mb();
>> > +	ioreq->state = STATE_IOREQ_INPROCESS;
>> > +
>> > +	mutex_lock(&kioreq->lock);
>> > +	list_for_each_entry(kioeventfd, &kioreq->ioeventfds, list) {
>> > +		if (ioreq->addr == kioeventfd->addr + VIRTIO_MMIO_QUEUE_NOTIFY &&
>> > +		    ioreq->size == kioeventfd->addr_len &&
>> > +		    (ioreq->data & QUEUE_NOTIFY_VQ_MASK) == kioeventfd->vq) {
>> > +			eventfd_signal(kioeventfd->eventfd, 1);
>> > +			state = STATE_IORESP_READY;
>> > +			break;
>> > +		}
>> > +	}
>> > +	mutex_unlock(&kioreq->lock);
>> > +
>> > +	smp_mb();
>> 
>> Is this really needed after calling mutex_unlock()? I think you are trying to
>> avoid any accesses to go past ioreq->state modification. If so, add a comment
>> (either why you need the barrier, or that you don't need it due to the unlock).
>
> Right, want all writes to finish before updating state.

I thought generally sync points act as full barriers. Doing a bunch of
grepping I think ends at:

  static __always_inline bool __mutex_unlock_fast(struct mutex *lock)
  {
          unsigned long curr = (unsigned long)current;

          return atomic_long_try_cmpxchg_release(&lock->owner, &curr, 0UL);
  }

so you should already have completed your writes by that point.

>
>> In general, shouldn't the state be checked and modified in the locked area?
>
> The handler runs separately for each vcpu and shouldn't run in parallel for the
> same vcpu. And so only one thread should ever be accessing ioreq port structure.
>
> The lock is there to protect the ioeventfds list (as mentioned in struct
> declaration) against parallel access, as threads for different vcpus may end up
> accessing it simultaneously.


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro