Date:   Fri, 1 Dec 2023 17:08:50 +0800
From:   Yi Liu <yi.l.liu@...el.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Nicolin Chen <nicolinc@...dia.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
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        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH v6 2/6] iommufd: Add IOMMU_HWPT_INVALIDATE

On 2023/12/1 15:10, Tian, Kevin wrote:
>> From: Liu, Yi L <yi.l.liu@...el.com>
>> Sent: Friday, December 1, 2023 3:05 PM
>>
>> On 2023/12/1 13:19, Tian, Kevin wrote:
>>>> From: Nicolin Chen <nicolinc@...dia.com>
>>>> Sent: Friday, December 1, 2023 12:50 PM
>>>>
>>>> On Fri, Dec 01, 2023 at 11:51:26AM +0800, Yi Liu wrote:
>>>>> On 2023/11/29 08:57, Jason Gunthorpe wrote:
>>>>>> On Tue, Nov 28, 2023 at 04:51:21PM -0800, Nicolin Chen wrote:
>>>>>>>>> I also thought about making this out_driver_error_code per HW.
>>>>>>>>> Yet, an error can be either per array or per entry/quest. The
>>>>>>>>> array-related error should be reported in the array structure
>>>>>>>>> that is a core uAPI, v.s. the per-HW entry structure. Though
>>>>>>>>> we could still report an array error in the entry structure
>>>>>>>>> at the first entry (or indexed by "array->entry_num")?
>>>>>>>>>
>>>>>>>>
>>>>>>>> why would there be an array error? array is just a software
>>>>>>>> entity containing actual HW invalidation cmds. If there is
>>>>>>>> any error with the array itself it should be reported via
>>>>>>>> ioctl errno.
>>>>>>>
>>>>>>> User array reading is a software operation, but kernel array
>>>>>>> reading is a hardware operation that can raise an error when
>>>>>>> the memory location to the array is incorrect or so.
>>>>>>
>>>>>> Well, we shouldn't get into a situation like that.. By the time the HW
>>>>>> got the address it should be valid.
>>>>>>
>>>>>>> With that being said, I think errno (-EIO) could do the job,
>>>>>>> as you suggested too.
>>>>>>
>>>>>> Do we have any idea what HW failures can be generated by the
>>>> commands
>>>>>> this will execture? IIRC I don't remember seeing any smmu specific
>>>>>> codes related to invalid invalidation? Everything is a valid input?
>>>>>>
>>>>>> Can vt-d fail single commands? What about AMD?
>>>>>
>>>>> Intel VT-d side, after each invalidation request, there is a wait
>>>>> descriptor which either provide an interrupt or an address for the
>>>>> hw to notify software the request before the wait descriptor has been
>>>>> completed. While, if there is error happened on the invalidation request,
>>>>> a flag (IQE, ICE, ITE) would be set in the Fault Status Register, and some
>>>>> detailed information would be recorded in the Invalidation Queue Error
>>>>> Record Register. So an invalidation request may be failed with some
>> error
>>>>> reported. If no error, will return completion via the wait descriptor. Is
>>>>> this what you mean by "fail a single command"?
>>>>
>>>> I see the current VT-d series marking those as "REVISIT". How
>>>> will it report an error to the user space from those register?
>>>>
>>>> Are they global status registers so that it might be difficult
>>>> to direct the error to the nested domain for an event fd?
>>>>
>>>
>>> They are global registers but invalidation queue is also the global
>>> resource. intel-iommu driver polls the status register after queueing
>>> new invalidation descriptors. The submission is serialized.
>>>
>>> If the error is related to a descriptor itself (e.g. format issue) then
>>> the head register points to the problematic descriptor so software
>>> can direct it to the related domain.
>>>
>>> If the error is related to device tlb invalidation (e.g. timeout) there
>>> is no way to associate the error with a specific descriptor by current
>>> spec. But intel-iommu driver batches descriptors per domain so
>>> we can still direct the error to the nested domain.
>>>
>>> But I don't see the need of doing it via eventfd.
>>>
>>> The poll semantics in intel-iommu driver is essentially a sync model.
>>> vt-d spec does allow software to optionally enable notification upon
>>> those errors but it's not used so far.
>>>
>>> With that I still prefer to having driver-specific error code defined
>>> in the entry. If ARM is an event-driven model then we can define
>>> that field at least in vtd specific data structure.
>>>
>>> btw given vtd doesn't use native format in uAPI it doesn't make
>>> sense to forward descriptor formatting errors back to userspace.
>>> Those, if happen, are driver's own problem. intel-iommu driver
>>> should verify the uAPI structure and return -EINVAL or proper
>>> errno to userspace purely in software.
>>>
>>> With that Yi please just define error codes for device tlb related
>>> errors for vtd.
>>
>> hmmm, this sounds like customized error code. is it? So far, VT-d
> 
> yes. there is no need to replicate hardware registers/bits if most
> of them are irrelevant to userspace.
> 
>> spec has two errors (ICE and ITE). ITE is valuable to let userspace
>> know. For ICE, looks like no much value. Intel iommu driver should
>> be responsible to submit a valid device-tlb invalidation to device.
> 
> it's an invalid completion message from the device which could be
> caused by various reasons (not exactly due to the invalidation
> request by iommu driver). so it still makes sense to forward.

ok. so we may need to define a field to forward the detailed info to
user as well. This data is error-code specific. @Nic, are we aligned
that the error_code field and error data reporting should be moved
to the driver-specific part since it is different between vendors?

-- 
Regards,
Yi Liu

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