Re: [PATCH v2 08/16] iommu: introduce device fault data

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Mon, 13 Nov 2017 13:06:24 +0000
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:

> On 10/11/17 22:18, Jacob Pan wrote:
> > On Fri, 10 Nov 2017 13:54:59 +0000
> > Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:
> >   
> >> On 09/11/17 19:36, Jacob Pan wrote:  
> >>> On Tue, 7 Nov 2017 11:38:50 +0000
> >>> Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:
> >>>     
> >>>> I think the IOMMU should pass the struct device associated to the
> >>>> BDF to the fault handler. The fault handler can then deduce the
> >>>> BDF from struct device if it needs to. This also allows to
> >>>> support faults from non-PCI devices, where the BDF or deviceID
> >>>> is specific to the IOMMU and doesn't mean anything to the device
> >>>> driver. 
> >>> Passing struct device is only useful if we use shared fault
> >>> notification method, as I did in V1 patch with group level or
> >>> current domain level.
> >>>
> >>> But the patch proposed here is a per device callback, there is no
> >>> need for passing struct device since it is implied.    
> >>
> >> Sorry I had lost sight of the original patch in this thread. I
> >> think the callback is fine as it is, in your patch:
> >>
> >> typedef int (*iommu_dev_fault_handler_t)(struct device *, struct
> >> iommu_fault_event *);
> >>  
> > I should have removed struct device here also. thanks for pointing
> > it out.  
> 
> Why remove it? The device driver will use a single C function as fault
> handler for multiple devices, so it needs struct device argument to
> understand the context.
> 
I meant to replace struct device * with just a void *, driver can
register fault callback with instance of their private data, this could
be a container struct of struct device.
e.g.
int iommu_register_device_fault_handler(struct device *dev,
				iommu_dev_fault_handler_t handler, void
				*data);

typedef int (*iommu_dev_fault_handler_t)(struct iommu_fault_event *, void *);


> [...]
> >>>  * @pasid: contains process address space ID, used in shared
> >>> virtual memory(SVM)
> >>>  * @rid: requestor ID
> >>>  * @page_req_group_id: page request group index
> >>>  * @last_req: last request in a page request group
> >>>  * @pasid_valid: indicates if the PRQ has a valid PASID
> >>>  * @prot: page access protection flag, e.g. IOMMU_READ,
> >>> IOMMU_WRITE    
> >>
> >> Should we also extend the prot flags then? PRI needs IOMMU_EXEC,
> >> IOMMU_PRIVILEGED. The problem with IOMMU_READ and IOMMU_WRITE is
> >> that it's not a bitfield, you can't OR values together. In order
> >> to extend it we need to change the value of IOMMU_READ to be 1 and
> >> IOMMU_WRITE to be 2. In PRI there is a case where R=W=0 (the PASID
> >> Stop marker), and we can't represent it with the existing
> >> IOMMU_READ value. 
> > don't we already have these in bit field? IOMMU_PRIV included. see
> > include/linux/iommu.h
> > #define IOMMU_READ	(1 << 0)
> > #define IOMMU_WRITE	(1 << 1)
> > #define IOMMU_CACHE	(1 << 2) /* DMA cache coherency */
> > #define IOMMU_NOEXEC	(1 << 3)
> > #define IOMMU_MMIO	(1 << 4) /* e.g. things like MSI
> > doorbells */ #define IOMMU_PRIV	(1 << 5)
> > #define IOMMU_EXEC	(1 << 6)  
> 
> Ah right, I was talking about IOMMU_FAULT_READ and IOMMU_FAULT_WRITE,
> sorry for the confusion. These flags are for creating mappings, they
> aren't really appropriate for fault reporting. What would the new
> IOMMU_EXEC flag mean in the context of iommu_map, which is already
> using IOMMU_NOEXEC (1 << 3)? What would IOMMU_CACHE and IOMMU_MMIO
> mean for fault reporting? It's probably easier to use a distinct set
> of flags for faults, by rewriting the IOMMU_FAULT_* flags.
> 
I see, it is in your patch i totally missed it. Make sense to me for
this separate set of flags.

> >>>  * @private_data: uniquely identify device-specific private data
> >>> for an
> >>>  *                individual page request    
>  [...]  
>  [...]  
> 
> That would work
> 
> >> For SMMU I've been abusing the private_data field to store
> >> SMMU-specific flags that can be used by the page_response handler
> >> to know how to send send the response:
> >>
> >> * Whether the fault was PRI or stall (the response command is
> >> different)
> >> * Whether the PRG response needs a PASID prefix or not. That's
> >> just a lazy shortcut and the property could be obtained
> >> differently. 
> > can you use pasid_valid bit for it?  
> 
> What I'm referring to is the "PRG Response PASID Required" bit in the
> PCI PRI capability, which is needed for the PRI response. I could dig
> it back from the struct device passed to the page response handler,
> but caching it in the private flags was more convenient. However I
> think I can get rid of the other flag PRI/stall by simply looking if
> struct device is a PCI dev. So we don't need iommu_private for the
> moment.
> 
> Thanks,
> Jean

[Jacob Pan]