Re: [PATCH v8 26/29] vfio-pci: Register an iommu fault handler

Auger Eric
2019-07-11T13:07:33Z

Hi Jean, Jacob,

On 6/18/19 4:04 PM, Jean-Philippe Brucker wrote:
> On 12/06/2019 19:53, Jacob Pan wrote:
>>>> You are right, the worst case of the spurious PS is to terminate the
>>>> group prematurely. Need to know the scope of the HW damage in case
>>>> of mdev where group IDs can be shared among mdevs belong to the
>>>> same PF.
>>>
>>> But from the IOMMU fault API point of view, the full page request is
>>> identified by both PRGI and PASID. Given that each mdev has its own
>>> set of PASIDs, it should be easy to isolate page responses per mdev.
>>>
>> On Intel platform, devices sending page request with private data must
>> receive page response with matching private data. If we solely depend
>> on PRGI and PASID, we may send stale private data to the device in
>> those incorrect page response. Since private data may represent PF
>> device wide contexts, the consequence of sending page response with
>> wrong private data may affect other mdev/PASID.
>>
>> One solution we are thinking to do is to inject the sequence #(e.g.
>> ktime raw mono clock) as vIOMMU private data into to the guest. Guest
>> would return this fake private data in page response, then host will
>> send page response back to the device that matches PRG1 and PASID and
>> private_data.
>>
>> This solution does not expose HW context related private data to the
>> guest but need to extend page response in iommu uapi.
>>
>> /**
>> * struct iommu_page_response - Generic page response information
>> * @version: API version of this structure
>> * @flags: encodes whether the corresponding fields are valid
>> * (IOMMU_FAULT_PAGE_RESPONSE_* values)
>> * @pasid: Process Address Space ID
>> * @grpid: Page Request Group Index
>> * @code: response code from &enum iommu_page_response_code
>> * @private_data: private data for the matching page request
>> */
>> struct iommu_page_response {
>> #define IOMMU_PAGE_RESP_VERSION_1 1
>> __u32 version;
>> #define IOMMU_PAGE_RESP_PASID_VALID (1 << 0)
>> #define IOMMU_PAGE_RESP_PRIVATE_DATA (1 << 1)
>> __u32 flags;
>> __u32 pasid;
>> __u32 grpid;
>> __u32 code;
>> __u32 padding;
>> __u64 private_data[2];
>> };
>>
>> There is also the change needed for separating storage for the real and
>> fake private data.
>>
>> Sorry for the last minute change, did not realize the HW implications.
>>
>> I see this as a future extension due to limited testing,
>
> I'm wondering how we deal with:
> (1) old userspace that won't fill the new private_data field in
> page_response. A new kernel still has to support it.
> (2) old kernel that won't recognize the new PRIVATE_DATA flag. Currently
> iommu_page_response() rejects page responses with unknown flags.
>
> I guess we'll need a two-way negotiation, where userspace queries
> whether the kernel supports the flag (2), and the kernel learns whether
> it should expect the private data to come back (1).
>
>> perhaps for
>> now, can you add paddings similar to page request? Make it 64B as well.
>
> I don't think padding is necessary, because iommu_page_response is sent
> by userspace to the kernel, unlike iommu_fault which is allocated by
> userspace and filled by the kernel.
>
> Page response looks a lot more like existing VFIO mechanisms, so I
> suppose we'll wrap the iommu_page_response structure and include an
> argsz parameter at the top:
>
> struct vfio_iommu_page_response {
> u32 argsz;
> struct iommu_page_response pr;
> };
>
> struct vfio_iommu_page_response vpr = {
> .argsz = sizeof(vpr),
> .pr = ...
> ...
> };
>
> ioctl(devfd, VFIO_IOMMU_PAGE_RESPONSE, &vpr);
>
> In that case supporting private data can be done by simply appending a
> field at the end (plus the negotiation above).

Sorry I did not quite follow the spurious response discussion but I just
noticed we still do have, upstream, in
iommu_unregister_device_fault_handler:

/* we cannot unregister handler if there are pending faults */
if (!list_empty(&param->fault_param->faults)) {
ret = -EBUSY;
goto unlock;
}

So did you eventually decide to let
iommu_unregister_device_fault_handler fail or is an oversight?

Thanks

Eric

>
> Thanks,
> Jean
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