Date:   Fri, 8 Dec 2017 13:59:09 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Lan Tianyu <tianyu.lan@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Jean Delvare <khali@...ux-fr.org>,
        "Christoph Hellwig" <hch@...radead.org>
Subject: Re: [PATCH v3 10/16] iommu: introduce device fault report API

On Fri, 8 Dec 2017 12:23:58 -0800
Jacob Pan <jacob.jun.pan@...ux.intel.com> wrote:

> On Thu, 7 Dec 2017 14:27:25 -0700
> Alex Williamson <alex.williamson@...hat.com> wrote:
> 
> > On Fri, 17 Nov 2017 10:55:08 -0800
> > Jacob Pan <jacob.jun.pan@...ux.intel.com> wrote:
> >   
> > > Traditionally, device specific faults are detected and handled
> > > within their own device drivers. When IOMMU is enabled, faults such
> > > as DMA related transactions are detected by IOMMU. There is no
> > > generic reporting mechanism to report faults back to the in-kernel
> > > device driver or the guest OS in case of assigned devices.
> > > 
> > > Faults detected by IOMMU is based on the transaction's source ID
> > > which can be reported at per device basis, regardless of the device
> > > type is a PCI device or not.
> > > 
> > > The fault types include recoverable (e.g. page request) and
> > > unrecoverable faults(e.g. access error). In most cases, faults can
> > > be handled by IOMMU drivers internally. The primary use cases are as
> > > follows:
> > > 1. page request fault originated from an SVM capable device that is
> > > assigned to guest via vIOMMU. In this case, the first level page
> > > tables are owned by the guest. Page request must be propagated to
> > > the guest to let guest OS fault in the pages then send page
> > > response. In this mechanism, the direct receiver of IOMMU fault
> > > notification is VFIO, which can relay notification events to QEMU
> > > or other user space software.
> > > 
> > > 2. faults need more subtle handling by device drivers. Other than
> > > simply invoke reset function, there are needs to let device driver
> > > handle the fault with a smaller impact.
> > > 
> > > This patchset is intended to create a generic fault report API such
> > > that it can scale as follows:
> > > - all IOMMU types
> > > - PCI and non-PCI devices
> > > - recoverable and unrecoverable faults
> > > - VFIO and other other in kernel users
> > > - DMA & IRQ remapping (TBD)
> > > The original idea was brought up by David Woodhouse and discussions
> > > summarized at https://lwn.net/Articles/608914/.
> > > 
> > > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > Signed-off-by: Ashok Raj <ashok.raj@...el.com>
> > > ---
> > >  drivers/iommu/iommu.c | 63
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++-
> > > include/linux/iommu.h | 36 +++++++++++++++++++++++++++++ 2 files
> > > changed, 98 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > > index 829e9e9..97b7990 100644
> > > --- a/drivers/iommu/iommu.c
> > > +++ b/drivers/iommu/iommu.c
> > > @@ -581,6 +581,12 @@ int iommu_group_add_device(struct iommu_group
> > > *group, struct device *dev) goto err_free_name;
> > >  	}
> > >  
> > > +	dev->iommu_param = kzalloc(sizeof(struct
> > > iommu_fault_param), GFP_KERNEL);
> > > +	if (!dev->iommu_param) {
> > > +		ret = -ENOMEM;
> > > +		goto err_free_name;
> > > +	}
> > > +
> > >  	kobject_get(group->devices_kobj);
> > >  
> > >  	dev->iommu_group = group;
> > > @@ -657,7 +663,7 @@ void iommu_group_remove_device(struct device
> > > *dev) sysfs_remove_link(&dev->kobj, "iommu_group");
> > >  
> > >  	trace_remove_device_from_group(group->id, dev);
> > > -
> > > +	kfree(dev->iommu_param);
> > >  	kfree(device->name);
> > >  	kfree(device);
> > >  	dev->iommu_group = NULL;
> > > @@ -791,6 +797,61 @@ int iommu_group_unregister_notifier(struct
> > > iommu_group *group, }
> > >  EXPORT_SYMBOL_GPL(iommu_group_unregister_notifier);
> > >  
> > > +int iommu_register_device_fault_handler(struct device *dev,
> > > +					iommu_dev_fault_handler_t
> > > handler,
> > > +					void *data)
> > > +{
> > > +	struct iommu_param *idata = dev->iommu_param;
> > > +
> > > +	/*
> > > +	 * Device iommu_param should have been allocated when
> > > device is
> > > +	 * added to its iommu_group.
> > > +	 */
> > > +	if (!idata)
> > > +		return -EINVAL;
> > > +	/* Only allow one fault handler registered for each device
> > > */
> > > +	if (idata->fault_param)
> > > +		return -EBUSY;
> > > +	get_device(dev);
> > > +	idata->fault_param =
> > > +		kzalloc(sizeof(struct iommu_fault_param),
> > > GFP_KERNEL);
> > > +	if (!idata->fault_param)
> > > +		return -ENOMEM;
> > > +	idata->fault_param->handler = handler;
> > > +	idata->fault_param->data = data;
> > > +
> > > +	return 0;
> > > +}
> > > +EXPORT_SYMBOL_GPL(iommu_register_device_fault_handler);
> > > +
> > > +int iommu_unregister_device_fault_handler(struct device *dev)
> > > +{
> > > +	struct iommu_param *idata = dev->iommu_param;
> > > +
> > > +	if (!idata)
> > > +		return -EINVAL;
> > > +
> > > +	kfree(idata->fault_param);
> > > +	idata->fault_param = NULL;
> > > +	put_device(dev);
> > > +
> > > +	return 0;
> > > +}
> > > +EXPORT_SYMBOL_GPL(iommu_unregister_device_fault_handler);
> > > +
> > > +
> > > +int iommu_report_device_fault(struct device *dev, struct
> > > iommu_fault_event *evt) +{
> > > +	/* we only report device fault if there is a handler
> > > registered */
> > > +	if (!dev->iommu_param || !dev->iommu_param->fault_param ||
> > > +		!dev->iommu_param->fault_param->handler)
> > > +		return -ENOSYS;
> > > +
> > > +	return dev->iommu_param->fault_param->handler(evt,
> > > +
> > > dev->iommu_param->fault_param->data); +}
> > > +EXPORT_SYMBOL_GPL(iommu_report_device_fault);
> > > +    
> > 
> > Isn't this all rather racy?  I see that we can have multiple callers
> > to register racing.  
> I agree, should use a lock here to prevent unregister. For multiple
> caller race, it won't happen since there is only one caller can
> register handler.

If you have multiple simultaneous callers to
iommu_register_device_fault_handler, they can all get past the test
for fault_param (testing and setting is not atomic), then it's
indeterminate which handler gets installed.  Thanks,

Alex

> >  Unregister is buggy, allowing any caller to
> > decrement the device reference regardless of whether there's one
> > outstanding through this interface.  The reporting callout can also
> > race with an unregistration.  Might need a mutex on iommu_param to
> > avoid.
> >   
> you are right, forgot to check outstanding handler. will add mutex also.
> 
> Thanks,
> > >  /**
> > >   * iommu_group_id - Return ID for a group
> > >   * @group: the group to ID
> > > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > > index dfda89b..841c044 100644
> > > --- a/include/linux/iommu.h
> > > +++ b/include/linux/iommu.h
> > > @@ -463,6 +463,14 @@ extern int
> > > iommu_group_register_notifier(struct iommu_group *group, struct
> > > notifier_block *nb); extern int
> > > iommu_group_unregister_notifier(struct iommu_group *group, struct
> > > notifier_block *nb); +extern int
> > > iommu_register_device_fault_handler(struct device *dev,
> > > +					iommu_dev_fault_handler_t
> > > handler,
> > > +					void *data);
> > > +
> > > +extern int iommu_unregister_device_fault_handler(struct device
> > > *dev); +
> > > +extern int iommu_report_device_fault(struct device *dev, struct
> > > iommu_fault_event *evt); +
> > >  extern int iommu_group_id(struct iommu_group *group);
> > >  extern struct iommu_group *iommu_group_get_for_dev(struct device
> > > *dev); extern struct iommu_domain
> > > *iommu_group_default_domain(struct iommu_group *); @@ -481,6
> > > +489,12 @@ extern void iommu_domain_window_disable(struct
> > > iommu_domain *domain, u32 wnd_nr) extern int
> > > report_iommu_fault(struct iommu_domain *domain, struct device *dev,
> > > unsigned long iova, int flags); +static inline bool
> > > iommu_has_device_fault_handler(struct device *dev) +{
> > > +	return dev->iommu_param && dev->iommu_param->fault_param &&
> > > +		dev->iommu_param->fault_param->handler;
> > > +}
> > > +    
> > 
> > This interface is racy by design, there's no guarantee that the
> > handler isn't immediately unregistered after this check. Thanks,
> >   
> right, I will fold this check into report function and protect by a
> lock. I was trying to save some cycles but it would not work with the
> race condition.
> > Alex
> >   
> > >  static inline void iommu_flush_tlb_all(struct iommu_domain *domain)
> > >  {
> > >  	if (domain->ops->flush_iotlb_all)
> > > @@ -734,6 +748,28 @@ static inline int
> > > iommu_group_unregister_notifier(struct iommu_group *group, return 0;
> > >  }
> > >  
> > > +static inline int iommu_register_device_fault_handler(struct
> > > device *dev,
> > > +
> > > iommu_dev_fault_handler_t handler,
> > > +						void *data)
> > > +{
> > > +	return 0;
> > > +}
> > > +
> > > +static inline int iommu_unregister_device_fault_handler(struct
> > > device *dev) +{
> > > +	return 0;
> > > +}
> > > +
> > > +static inline bool iommu_has_device_fault_handler(struct device
> > > *dev) +{
> > > +	return false;
> > > +}
> > > +
> > > +static inline int iommu_report_device_fault(struct device *dev,
> > > struct iommu_fault_event *evt) +{
> > > +	return 0;
> > > +}
> > > +
> > >  static inline int iommu_group_id(struct iommu_group *group)
> > >  {
> > >  	return -ENODEV;    
> >   
> 
> [Jacob Pan]

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