linux-kernel - Re: [PATCH v6 2/6] iommufd: Add IOMMU_HWPT

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Message-ID: <51640965-4196-4da1-88b4-cb0e406931f3@intel.com>
Date:   Tue, 28 Nov 2023 14:01:59 +0800
From:   Yi Liu <yi.l.liu@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "Zeng, Xin" <xin.zeng@...el.com>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH v6 2/6] iommufd: Add IOMMU_HWPT_INVALIDATE

On 2023/11/28 03:53, Nicolin Chen wrote:
> On Fri, Nov 24, 2023 at 02:36:29AM +0000, Tian, Kevin wrote:
> 
>>>>>>>>> + * @out_driver_error_code: Report a driver speicifc error code
>>> upon
>>>>>>> failure.
>>>>>>>>> + *                         It's optional, driver has a choice to fill it or
>>>>>>>>> + *                         not.
>>>>>>>>
>>>>>>>> Being optional how does the user tell whether the code is filled or
>>> not?
>>>>>
>>>>> Well, naming it "error_code" indicates zero means no error while
>>>>> non-zero means something? An error return from this ioctl could
>>>>> also tell the user space to look up for this driver error code,
>>>>> if it ever cares.
>>>>
>>>> probably over-thinking but I'm not sure whether zero is guaranteed to
>>>> mean no error in all implementations...
>>>
>>> Well, you are right. Usually HW conveniently raises a flag in a
>>> register to indicate something wrong, yet it is probably unsafe
>>> to say it definitely.
>>>
>>
>> this reminds me one open. What about an implementation having
>> a hierarchical error code layout e.g. one main error register with
>> each bit representing an error category then multiple error code
>> registers each for one error category? In this case probably
>> a single out_driver_error_code cannot carry that raw information.
> 
> Hmm, good point.
> 
>> Instead the iommu driver may need to define a customized error
>> code convention in uapi header which is converted from the
>> raw error information.
>>
>>  From this angle should we simply say that the error code definition
>> must be included in the uapi header? If raw error information can
>> be carried by this field then this hw can simply say that the error
>> code format is same as the hw spec defines.
>>
>> With that explicit information then the viommu can easily tell
>> whether error code is filled or not based on its own convention.
> 
> That'd be to put this error_code field into the driver uAPI
> structure right?

looks to be. Then it would be convenient to reserve a code for
the case of no error (either no error happened or just not used)

> 
> 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")?

per-entry error code seems like to be a completion code. Each
entry in the array can have a corresponding code (0 for succ,
others for failure). do you already have such a need?

-- 
Regards,
Yi Liu