Re: [PATCH v5 15/23] iommu: handle page response timeout

Jacob Pan
2018-05-29T16:18:05Z

On Mon, 14 May 2018 15:43:54 +0800
Lu Baolu <baolu.lu@linux.intel.com> wrote:

> Hi,
>
> On 05/12/2018 04:54 AM, 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 | 53
> > +++++++++++++++++++++++++++++++++++++++++++++++++++
> > include/linux/iommu.h | 4 ++++ 2 files changed, 57 insertions(+)
> >
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 02fed3e..1f2f49e 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -827,6 +827,37 @@ int iommu_group_unregister_notifier(struct
> > iommu_group *group, }
> > EXPORT_SYMBOL_GPL(iommu_group_unregister_notifier);
> >
> > +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;
> > +
> > + 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.
> > + * 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(evt, &fparam->faults, list) {
> > + if (time_after64(now, evt->expire))
> > + 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 + prq_timeout);
> > +}
> > +
>
> This timer scheme is very rough.
>
yes, the timer is a rough safety net for misbehaved PRQ handlers such
as a guest.
> The timer expires every 10 seconds (by default).
>
> 0 10 20
> 30 40
> +---------------+---------------+---------------+---------------+ ^
> ^ ^ ^ ^ | | |
> | | F0 F1 F2 F3
> (F1,F2,F3 will not be handled until here!)
>
> F0, F1, F2, F3 are four page faults happens during [0, 10s) time
> window. F1, F2, F3 timeout won't be handled until the timer expires
> again at 20s. That means a fault might be pending there until about
> (2 * prq_timeout) seconds later.
>
correct. it could be 2x for the worst case. I should explain in
comments.
> Out of curiosity, Why not adding a timer in iommu_fault_event,
> starting it in iommu_report_device_fault() and removing it in
> iommu_page_response()?
>
I thought about that also but since we are just trying to have a broad
and rough safety net (in addition to potential HW mechanism or credit
based solution), my thought was that having a per device timer is more
economical than per event.
Thanks for the in-depth check!

> Best regards,
> Lu Baolu
>
>
> [...]
>

[Jacob Pan]