Re: Plan for /dev/ioasid RFC v2

Eric Auger
2021-06-09T10:15:00Z

Hi Kevin,

On 6/9/21 11:37 AM, Tian, Kevin wrote:
>> From: Eric Auger <eric.auger@redhat.com>
>> Sent: Wednesday, June 9, 2021 4:15 PM
>>
>> Hi Kevin,
>>
>> On 6/7/21 4:58 AM, Tian, Kevin wrote:
>>> Hi, all,
>>>
>>> We plan to work on v2 now, given many good comments already received
>>> and substantial changes envisioned. This is a very complex topic with
>>> many sub-threads being discussed. To ensure that I didn't miss valuable
>>> suggestions (and also keep everyone on the same page), here I'd like to
>>> provide a list of planned changes in my mind. Please let me know if
>>> anything important is lost. :)
>>>
>>> --
>>>
>>> (Remaining opens in v1)
>>>
>>> - Protocol between kvm/vfio/ioasid for wbinvd/no-snoop. I'll see how
>>> much can be refined based on discussion progress when v2 is out;
>>>
>>> - Device-centric (Jason) vs. group-centric (David) uAPI. David is not fully
>>> convinced yet. Based on discussion v2 will continue to have ioasid uAPI
>>> being device-centric (but it's fine for vfio to be group-centric). A new
>>> section will be added to elaborate this part;
>>>
>>> - PASID virtualization (section 4) has not been thoroughly discussed yet.
>>> Jason gave some suggestion on how to categorize intended usages.
>>> I will rephrase this section and hope more discussions can be held for
>>> it in v2;
>>>
>>> (Adopted suggestions)
>>>
>>> - (Jason) Rename /dev/ioasid to /dev/iommu (so does uAPI e.g. IOASID
>>> _XXX to IOMMU_XXX). One suggestion (Jason) was to also rename
>>> RID+PASID to SID+SSID. But given the familiarity of the former, I will
>>> still use RID+PASID in v2 to ease the discussoin;
>>>
>>> - (Jason) v1 prevents one device from binding to multiple ioasid_fd's. This
>>> will be fixed in v2;
>>>
>>> - (Jean/Jason) No need to track guest I/O page tables on ARM/AMD.
>> When
>>> a pasid table is bound, it becomes a container for all guest I/O page
>> tables;
>> while I am totally in line with that change, I guess we need to revisit
>> the invalidate ioctl
>> to support PASID table invalidation.
> Yes, this is planned when doing this change.
OK
>
>>> - (Jean/Jason) Accordingly a device label is required so iotlb invalidation
>>> and fault handling can both support per-device operation. Per Jean's
>>> suggestion, this label will come from userspace (when VFIO_BIND_
>>> IOASID_FD);
>> what is not totally clear to me is the correspondance between this label
>> and the SID/SSID tuple.
>> My understanding is it rather maps to the SID because you can attach
>> several ioasids to the device.
>> So it is not clear to me how you reconstruct the SSID info
>>
> Yes, device handle maps to SID. The fault data reported to userspace
> will include {device_label, ioasid, vendor_fault_data}. In your case
> I believe SSID will be included in vendor_fault_data thus no reconstruct
> required. For Intel the user could figure out vPASID according to device_
> label and ioasid, i.e. no need to include PASID info in vendor_fault_data.
OK that works.

Thanks

Eric
>
> Thanks
> Kevin