Re: [RFC PATCH v3 1/8] iommu: Evolve the device fault reporting framework

[Shenming Lu]

On 2021/5/19 2:58, Alex Williamson wrote:
> On Fri, 9 Apr 2021 11:44:13 +0800
> Shenming Lu <lushenming@xxxxxxxxxx> wrote:
> 
>> This patch follows the discussion here:
>>
>> https://lore.kernel.org/linux-acpi/YAaxjmJW+ZMvrhac@myrica/
>>
>> Besides SVA/vSVA, such as VFIO may also enable (2nd level) IOPF to remove
>> pinning restriction. In order to better support more scenarios of using
>> device faults, we extend iommu_register_fault_handler() with flags and
>> introduce FAULT_REPORT_ to describe the device fault reporting capability
>> under a specific configuration.
>>
>> Note that we don't further distinguish recoverable and unrecoverable faults
>> by flags in the fault reporting cap, having PAGE_FAULT_REPORT_ +
>> UNRECOV_FAULT_REPORT_ seems not a clean way.
>>
>> In addition, still take VFIO as an example, in nested mode, the 1st level
>> and 2nd level fault reporting may be configured separately and currently
>> each device can only register one iommu dev fault handler, so we add a
>> handler update interface for this.
> 
> 
> IIUC, you're introducing flags for the fault handler callout, which
> essentially allows the IOMMU layer to filter which types of faults the
> handler can receive.  You then need an update function to modify those
> flags.  Why can't the handler itself perform this filtering?  For
> instance in your vfio example, the handler registered by the type1
> backend could itself return fault until the fault transfer path to the
> device driver is established.  Thanks,

As discussed in [1]:

In nested IOPF, we have to figure out whether a fault comes from L1 or L2.
A SMMU stall event comes with this information, but a PRI page request doesn't.
The IOMMU driver can walk the page tables to find out the level information.
If the walk terminates at L1, further inject to the guest. Otherwise fix the
fault at L2 in VFIO. It's not efficient compared to hardware-provided info.

And in pinned case, if VFIO can tell the IOMMU driver that no L2 fault is
expected, there is no need to walk the page tables in the IOMMU driver?

[1] https://patchwork.kernel.org/project/linux-arm-kernel/patch/20210108145217.2254447-4-jean-philippe@xxxxxxxxxx/

Thanks,
Shenming

> 
> Alex
> 
> .
>