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Date:   Wed, 25 Apr 2018 08:37:11 -0700
From:   Jacob Pan <jacob.jun.pan@linux.intel.com>
To:     Jean-Philippe Brucker <Jean-Philippe.Brucker@arm.com>
Cc:     "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        jacob.jun.pan@linux.intel.com
Subject: Re: [PATCH v4 14/22] iommu: handle page response timeout
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On Mon, 23 Apr 2018 16:36:23 +0100
Jean-Philippe Brucker <Jean-Philippe.Brucker@arm.com> wrote:

> On Mon, Apr 16, 2018 at 10:49:03PM +0100, Jacob Pan wrote:
> > When IO page faults are reported outside IOMMU subsystem, the page
> > request handler may fail for various reasons. E.g. a guest received
> > page requests but did not have a chance to run for a long time. The
> > irresponsive behavior could hold off limited resources on the
> > pending device.
> > There can be hardware or credit based software solutions as
> > suggested in the PCI ATS Ch-4. To provide a basic safty net this
> > patch introduces a per device deferrable timer which monitors the
> > longest pending page fault that requires a response. Proper action
> > such as sending failure response code could be taken when timer
> > expires but not included in this patch. We need to consider the
> > life cycle of page groupd ID to prevent confusion with reused group
> > ID by a device. For now, a warning message provides clue of such
> > failure.
> > 
> > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > Signed-off-by: Ashok Raj <ashok.raj@intel.com>
> > ---
> >  drivers/iommu/iommu.c | 60
> > +++++++++++++++++++++++++++++++++++++++++++++++++--
> > include/linux/iommu.h |  4 ++++ 2 files changed, 62 insertions(+),
> > 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 628346c..f6512692 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -799,6 +799,39 @@ int iommu_group_unregister_notifier(struct
> > iommu_group *group, }
> >  EXPORT_SYMBOL_GPL(iommu_group_unregister_notifier);
> >  
> > +/* Max time to wait for a pending page request */
> > +#define IOMMU_PAGE_RESPONSE_MAXTIME (HZ * 10)
> > +static void iommu_dev_fault_timer_fn(struct timer_list *t)
> > +{
> > +	struct iommu_fault_param *fparam = from_timer(fparam, t,
> > timer);
> > +	struct iommu_fault_event *evt, *iter;
> > +
> > +	u64 now;
> > +
> > +	now = get_jiffies_64();
> > +
> > +	/* The goal is to ensure driver or guest page fault
> > handler(via vfio)
> > +	 * send page response on time. Otherwise, limited queue
> > resources
> > +	 * may be occupied by some irresponsive guests or
> > drivers.  
> 
> By "limited queue resources", do you mean the PRI fault queue in the
> pIOMMU device, or something else?
> 
I am referring to the device resource for tracking pending PRQs. Intel
IOMMU does not track pending PRQs.
> 
> I'm still uneasy about this timeout. We don't really know if the guest
> doesn't respond because it is suspended, because it doesn't support
> PRI or because it's attempting to kill the host. In the first case,
> then receiving and responding to page requests later than 10s should
> be fine, right?
> 
when a guest is going into system suspend, suspend callback functions of
assigned device driver and vIOMMU should be called. I think vIOMMU
should propagate the iommu_suspend call to host IOMMU driver, therefore
terminate all the pending PRQs. We can make the timeout adjustable.

> Or maybe the guest is doing something weird like fetching pages from
> network storage and it occasionally hits a latency oddity. This
> wouldn't interrupt the fault queues, because other page requests for
> the same device can be serviced in parallel, but if you implement a
> PRG timeout it would still unfairly disable PRI.
> 
The timeout here is intended to be a broader and basic safety net at
per device level. We can implement finer grain safety mechanism but I
am guessing it is better to be done in HW.
> In the other cases (unsupported PRI or rogue guest) then disabling PRI
> using a FAILURE status might be the right thing to do. However,
> assuming the device follows the PCI spec it will stop sending page
> requests once there are as many PPRs in flight as the allocated
> credit.
> 
Agreed, here I am not taking any actions. There may be need to drain
in-fly requests.
> Even though drivers set the PPR credit number arbitrarily (because
> finding an ideal number is difficult or impossible), the device stops
> issuing faults at some point if the guest is unresponsive, and it
> won't grab any more shared resources, or use slots in shared queues.
> Resources for pending faults can be cleaned when the device is reset
> and assigned to a different guest.
> 
> 
> That's for sane endpoints that follow the spec. If on the other hand,
> we can't rely on the device implementation to respect our maximum
> credit allocation, then we should do the accounting ourselves and
> reject incoming faults with INVALID as fast as possible. Otherwise
> it's an easy way for a guest to DoS the host and I don't think a
> timeout solves this problem (The guest can wait 9 seconds before
> replying to faults and meanwhile fill all the queues). In addition
> the timeout is done on PRGs but not individual page faults, so a
> guest could overflow the queues by triggering lots of page requests
> without setting the last bit.
> 
> 
> If there isn't any possibility of memory leak or abusing resources, I
> don't think it's our problem that the guest is excessively slow at
> handling page requests. Setting an upper bound to page request latency
> might do more harm than good. Ensuring that devices respect the number
> of allocated in-flight PPRs is more important in my opinion.
> 
How about we have a really long timeout, e.g. 1 min similar to device
invalidate response timeout in ATS spec., just for basic safety and
diagnosis. Optionally, we could have quota in parallel.

> > +	 * When per device pending fault list is not empty, we
> > periodically checks
> > +	 * if any anticipated page response time has expired.
> > +	 *
> > +	 * TODO:
> > +	 * We could do the following if response time expires:
> > +	 * 1. send page response code FAILURE to all pending PRQ
> > +	 * 2. inform device driver or vfio
> > +	 * 3. drain in-flight page requests and responses for this
> > device
> > +	 * 4. clear pending fault list such that driver can
> > unregister fault
> > +	 *    handler(otherwise blocked when pending faults are
> > present).
> > +	 */
> > +	list_for_each_entry_safe(evt, iter, &fparam->faults, list)
> > {
> > +		if (time_after64(evt->expire, now))
> > +			pr_err("Page response time expired!, pasid
> > %d gid %d exp %llu now %llu\n",
> > +				evt->pasid,
> > evt->page_req_group_id, evt->expire, now);
> > +	}
> > +	mod_timer(t, now + IOMMU_PAGE_RESPONSE_MAXTIME);
> > +}
> > +
> >  /**
> >   * iommu_register_device_fault_handler() - Register a device fault
> > handler
> >   * @dev: the device
> > @@ -806,8 +839,8 @@
> > EXPORT_SYMBOL_GPL(iommu_group_unregister_notifier);
> >   * @data: private data passed as argument to the handler
> >   *
> >   * When an IOMMU fault event is received, call this handler with
> > the fault event
> > - * and data as argument. The handler should return 0. If the fault
> > is
> > - * recoverable (IOMMU_FAULT_PAGE_REQ), the handler must also
> > complete
> > + * and data as argument. The handler should return 0 on success.
> > If the fault is
> > + * recoverable (IOMMU_FAULT_PAGE_REQ), the handler can also
> > complete  
> 
> This change might belong in patch 12/22
> 
Good point, will fix
> >   * the fault by calling iommu_page_response() with one of the
> > following
> >   * response code:
> >   * - IOMMU_PAGE_RESP_SUCCESS: retry the translation
> > @@ -848,6 +881,9 @@ int iommu_register_device_fault_handler(struct
> > device *dev, param->fault_param->data = data;
> >  	INIT_LIST_HEAD(&param->fault_param->faults);
> >  
> > +	timer_setup(&param->fault_param->timer,
> > iommu_dev_fault_timer_fn,
> > +		TIMER_DEFERRABLE);
> > +
> >  	mutex_unlock(&param->lock);
> >  
> >  	return 0;
> > @@ -905,6 +941,8 @@ int iommu_report_device_fault(struct device
> > *dev, struct iommu_fault_event *evt) {
> >  	int ret = 0;
> >  	struct iommu_fault_event *evt_pending;
> > +	struct timer_list *tmr;
> > +	u64 exp;
> >  	struct iommu_fault_param *fparam;
> >  
> >  	/* iommu_param is allocated when device is added to group
> > */ @@ -925,6 +963,17 @@ int iommu_report_device_fault(struct device
> > *dev, struct iommu_fault_event *evt) goto done_unlock;
> >  		}
> >  		memcpy(evt_pending, evt, sizeof(struct
> > iommu_fault_event));
> > +		/* Keep track of response expiration time */
> > +		exp = get_jiffies_64() +
> > IOMMU_PAGE_RESPONSE_MAXTIME;
> > +		evt_pending->expire = exp;
> > +
> > +		if (list_empty(&fparam->faults)) {  
> 
> The list_empty() and timer modification need to be inside
> fparam->lock, otherwise we race with iommu_page_response
> 
right, thanks.
> Thanks,
> Jean
> 
> > +			/* First pending event, start timer */
> > +			tmr =
> > &dev->iommu_param->fault_param->timer;
> > +			WARN_ON(timer_pending(tmr));
> > +			mod_timer(tmr, exp);
> > +		}
> > +
> >  		mutex_lock(&fparam->lock);
> >  		list_add_tail(&evt_pending->list, &fparam->faults);
> >  		mutex_unlock(&fparam->lock);
> > @@ -1542,6 +1591,13 @@ int iommu_page_response(struct device *dev,
> >  		}
> >  	}
> >  
> > +	/* stop response timer if no more pending request */
> > +	if (list_empty(&param->fault_param->faults) &&
> > +		timer_pending(&param->fault_param->timer)) {
> > +		pr_debug("no pending PRQ, stop timer\n");
> > +		del_timer(&param->fault_param->timer);
> > +	}  

[Jacob Pan]