Re: [PATCH v2 10/16] iommu: introduce device fault report API

[Jacob Pan]

On Fri, 6 Oct 2017 10:36:02 +0100
Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx> wrote:

> Hi Jacob,
> 
> On 06/10/17 00:03, Jacob Pan 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> > ---  
> [...]
> > +int iommu_register_device_fault_handler(struct device *dev,
> > +					iommu_dev_fault_handler_t
> > handler) +{
> > +	if (dev->iommu_fault_param)
> > +		return -EBUSY;
> > +	get_device(dev);
> > +	dev->iommu_fault_param =
> > +		kzalloc(sizeof(struct iommu_fault_param),
> > GFP_KERNEL);
> > +	if (!dev->iommu_fault_param)
> > +		return -ENOMEM;
> > +	dev->iommu_fault_param->dev_fault_handler = handler;  
> 
> Since the handler is owned by a device driver, you also need to clean
> it up when switching the driver (native->VFIO and VFIO->native), in
> iommu_attach_device I suppose.
> 
I was thinking the driver who registered fault handler shall be held
accountable to unregister. e.g. User must unbind driver (unregister
fault handler included) before assigning device to vfio-pci. Otherwise,
VFIO call to register handler would fail.
I am assuming VFIO needs to have a separate device fault handler of its
own.

Jacob