Re: [PATCH 2/4] iommu: Introduce device fault data

[Jean-Philippe Brucker <jean-philippe.brucker@arm.com>]

On 24/05/2019 14:49, Jacob Pan wrote:
> On Thu, 23 May 2019 19:43:46 +0100
> Robin Murphy <robin.murphy@arm.com> wrote:
>>> +/**
>>> + * struct iommu_fault_event - Generic fault event
>>> + *
>>> + * Can represent recoverable faults such as a page requests or
>>> + * unrecoverable faults such as DMA or IRQ remapping faults.
>>> + *
>>> + * @fault: fault descriptor
>>> + * @iommu_private: used by the IOMMU driver for storing
>>> fault-specific
>>> + *                 data. Users should not modify this field before
>>> + *                 sending the fault response.  
>>
>> Sorry if I'm a bit late to the party, but given that description, if 
>> users aren't allowed to touch this then why expose it to them at all? 
>> I.e. why not have iommu_report_device_fault() pass just the fault
>> itself to the fault handler:
>>
>> 	ret = fparam->handler(&evt->fault, fparam->data);
>>
>> and let the IOMMU core/drivers decapsulate it again later if need be. 
>> AFAICS drivers could also just embed the entire generic event in
>> their own private structure anyway, just as we do for domains.
>>
> I can't remember all the discussion history but I think iommu_private
> is used similarly to the page request private data (device private).

Hm yes, we already have iommu_fault_page_request::private_data for that.
I think I used to stash flags in iommu_private (is_stall and
needs_pasid), so that the SMMUv3 driver doesn't need to go fetch them
from the device structure, but I removed them. If VT-d doesn't need
iommu_private either, maybe we can remove it entirely?

In any case I agree that device drivers should only need to know about
evt->fault.

> We
> need to inject the data to the guest and the guest will send the
> unmodified data back along with response.

By the way, does private_data need to go back through the
iommu_page_response() path? The current series doesn't do that.

> The private data can be used
> to tag internal device/iommu context.

> I think we can do the way you said by keeping them within iommu core
> and recover it based on the response but that would require tracking
> each fault report, right?

That's already the case: we decided in thread [1] to track recoverable
faults in the IOMMU core, in order to check that the response is sane
and to set a quota and/or timeout. (I didn't include your timeout
patches here because I think they need a little more work. They are on
my sva/api branch.)

I already dropped iommu_private from the iommu_page_response structure.
In patch 4 iommu_page_response() retrieves the fault event and pass the
corresponding iommu_private back to the IOMMU driver.

[1] https://lore.kernel.org/lkml/20171206112521.1edf8e9b@jacob-builder/

Thanks,
Jean

> 
> If we pass on the private data, we only need to check if the response
> belong to the device but not exact match of a specific fault since the
> damage is contained in the assigned device. In case of injection
> fault into the guest, the response will come asynchronously after the
> handler completes.