Re: [PATCH v4 23/26] iommu/arm-smmu-v3: Add stall support for platform devices

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 4 Mar 2020 15:08:33 +0100
Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:

> On Thu, Feb 27, 2020 at 06:17:26PM +0000, Jonathan Cameron wrote:
> > On Mon, 24 Feb 2020 19:23:58 +0100
> > Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:
> >   
> > > From: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> > > 
> > > The SMMU provides a Stall model for handling page faults in platform
> > > devices. It is similar to PCI PRI, but doesn't require devices to have
> > > their own translation cache. Instead, faulting transactions are parked and
> > > the OS is given a chance to fix the page tables and retry the transaction.
> > > 
> > > Enable stall for devices that support it (opt-in by firmware). When an
> > > event corresponds to a translation error, call the IOMMU fault handler. If
> > > the fault is recoverable, it will call us back to terminate or continue
> > > the stall.
> > > 
> > > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>  
> > One question inline.
> > 
> > Thanks,
> >   
> > > ---
> > >  drivers/iommu/arm-smmu-v3.c | 271 ++++++++++++++++++++++++++++++++++--
> > >  drivers/iommu/of_iommu.c    |   5 +-
> > >  include/linux/iommu.h       |   2 +
> > >  3 files changed, 269 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> > > index 6a5987cce03f..da5dda5ba26a 100644
> > > --- a/drivers/iommu/arm-smmu-v3.c
> > > +++ b/drivers/iommu/arm-smmu-v3.c
> > > @@ -374,6 +374,13 @@  
> > 
> >   
> > > +/*
> > > + * arm_smmu_flush_evtq - wait until all events currently in the queue have been
> > > + *                       consumed.
> > > + *
> > > + * Wait until the evtq thread finished a batch, or until the queue is empty.
> > > + * Note that we don't handle overflows on q->batch. If it occurs, just wait for
> > > + * the queue to be empty.
> > > + */
> > > +static int arm_smmu_flush_evtq(void *cookie, struct device *dev, int pasid)
> > > +{
> > > +	int ret;
> > > +	u64 batch;
> > > +	struct arm_smmu_device *smmu = cookie;
> > > +	struct arm_smmu_queue *q = &smmu->evtq.q;
> > > +
> > > +	spin_lock(&q->wq.lock);
> > > +	if (queue_sync_prod_in(q) == -EOVERFLOW)
> > > +		dev_err(smmu->dev, "evtq overflow detected -- requests lost\n");
> > > +
> > > +	batch = q->batch;  
> > 
> > So this is trying to be sure we have advanced the queue 2 spots?  
> 
> So we call arm_smmu_flush_evtq() before decommissioning a PASID, to make
> sure that there aren't any pending event for this PASID languishing in the
> fault queues.
> 
> The main test is queue_empty(). If that succeeds then we know that there
> aren't any pending event (and the PASID is safe to reuse). But if new
> events are constantly added to the queue then we wait for the evtq thread
> to handle a full batch, where one batch corresponds to the queue size. For
> that we take the batch number when entering flush(), and wait for the evtq
> thread to increment it twice.
> 
> > Is there a potential race here?  q->batch could have updated before we take
> > a local copy.  
> 
> Yes we're just checking on the progress of the evtq thread. All accesses
> to batch are made while holding the wq lock.
> 
> Flush is a rare event so the lock isn't contended, but the wake_up() that
> this patch introduces in arm_smmu_evtq_thread() does add some overhead
> (0.85% of arm_smmu_evtq_thread(), according to perf). It would be nice to
> get rid of it but I haven't found anything clever yet.
> 

Thanks.  Maybe worth a few comments in the code as this is a bit esoteric.

Thanks,

Jonathan

> Thanks,
> Jean
> 
> >   
> > > +	ret = wait_event_interruptible_locked(q->wq, queue_empty(&q->llq) ||
> > > +					      q->batch >= batch + 2);
> > > +	spin_unlock(&q->wq.lock);
> > > +
> > > +	return ret;
> > > +}
> > > +  
> > ...
> >   



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