linux-kernel - Re: [PATCH v4 01/17] iommu: Add hwpt_type with user_data for domain_alloc

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Date:   Mon, 25 Sep 2023 14:17:37 +0800
From:   Yi Liu <yi.l.liu@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Baolu Lu <baolu.lu@...ux.intel.com>
CC:     <joro@...tes.org>, <alex.williamson@...hat.com>,
        <kevin.tian@...el.com>, <cohuck@...hat.com>,
        <eric.auger@...hat.com>, <nicolinc@...dia.com>,
        <kvm@...r.kernel.org>, <mjrosato@...ux.ibm.com>,
        <chao.p.peng@...ux.intel.com>, <yi.y.sun@...ux.intel.com>,
        <peterx@...hat.com>, <jasowang@...hat.com>,
        <shameerali.kolothum.thodi@...wei.com>, <lulu@...hat.com>,
        <suravee.suthikulpanit@....com>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <zhenzhong.duan@...el.com>, <joao.m.martins@...cle.com>
Subject: Re: [PATCH v4 01/17] iommu: Add hwpt_type with user_data for
 domain_alloc_user op

On 2023/9/22 17:47, Robin Murphy wrote:
> On 2023-09-21 17:44, Jason Gunthorpe wrote:
>> On Thu, Sep 21, 2023 at 08:12:03PM +0800, Baolu Lu wrote:
>>> On 2023/9/21 15:51, Yi Liu wrote:
>>>> diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
>>>> index 4a7c5c8fdbb4..3c8660fe9bb1 100644
>>>> --- a/include/uapi/linux/iommufd.h
>>>> +++ b/include/uapi/linux/iommufd.h
>>>> @@ -357,6 +357,14 @@ enum iommufd_hwpt_alloc_flags {
>>>>        IOMMU_HWPT_ALLOC_NEST_PARENT = 1 << 0,
>>>>    };
>>>> +/**
>>>> + * enum iommu_hwpt_type - IOMMU HWPT Type
>>>> + * @IOMMU_HWPT_TYPE_DEFAULT: default
>>>
>>> How about s/default/vendor agnostic/ ?
>>
>> Please don't use the word vendor :)
>>
>> IOMMU_HWPT_TYPE_GENERIC perhaps if we don't like default
> 
> Ah yes, a default domain type, not to be confused with any default domain 
> type, including the default default domain type. Just in case anyone had 
> forgotten how gleefully fun this is :D
> 
> I particularly like the bit where we end up with this construct later:
> 
>      switch (hwpt_type) {
>      case IOMMU_HWPT_TYPE_DEFAULT:
>          /* allocate a domain */
>      default:
>          /* allocate a different domain */
>      }
> 
> But of course neither case allocates a *default* domain, because it's quite 
> obviously the wrong place to be doing that.
> 
> I could go on enjoying myself, but basically yeah, "default" can't be a 
> type in itself (at best it would be a meta-type which could be requested, 
> such that it resolves to some real type to actually allocate), so a good 
> name should reflect what the type functionally *means* to the user. IIUC 
> the important distinction is that it's an abstract kernel-owned pagetable 
> for the user to indirectly control via the API, rather than one it owns and 
> writes directly (and thus has to be in a specific agreed format).

yes. It is just what the existing domain_alloc_user op does. Here we add
a hwpt_type as the type can be given by user, so we need to define a 
specific type for it.

Perhaps we can also name it as IOMMU_HWPT_TYPE_UNMANAGED to be aligned with
the domain type naming. IOMMU_HWPT_TYPE_GENERIC is also a good choice.
Please feel free let me know your preference.

-- 
Regards,
Yi Liu