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Subject: Re: [RFC] /dev/ioasid uAPI proposal
To:     "Tian, Kevin" <kevin.tian@intel.com>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Liu Yi L <yi.l.liu@linux.intel.com>
Cc:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Alex Williamson (alex.williamson@redhat.com)\"\"" 
        <alex.williamson@redhat.com>, Jason Gunthorpe <jgg@nvidia.com>,
        David Woodhouse <dwmw2@infradead.org>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <42a71462-1abc-0404-156c-60a7ee1ad333@redhat.com>
Date:   Tue, 1 Jun 2021 16:47:15 +0800
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在 2021/6/1 下午2:16, Tian, Kevin 写道:
>> From: Jason Wang
>> Sent: Tuesday, June 1, 2021 2:07 PM
>>
>> 在 2021/6/1 下午1:42, Tian, Kevin 写道:
>>>> From: Jason Wang
>>>> Sent: Tuesday, June 1, 2021 1:30 PM
>>>>
>>>> 在 2021/6/1 下午1:23, Lu Baolu 写道:
>>>>> Hi Jason W,
>>>>>
>>>>> On 6/1/21 1:08 PM, Jason Wang wrote:
>>>>>>>> 2) If yes, what's the reason for not simply use the fd opened from
>>>>>>>> /dev/ioas. (This is the question that is not answered) and what
>>>>>>>> happens
>>>>>>>> if we call GET_INFO for the ioasid_fd?
>>>>>>>> 3) If not, how GET_INFO work?
>>>>>>> oh, missed this question in prior reply. Personally, no special reason
>>>>>>> yet. But using ID may give us opportunity to customize the
>> management
>>>>>>> of the handle. For one, better lookup efficiency by using xarray to
>>>>>>> store the allocated IDs. For two, could categorize the allocated IDs
>>>>>>> (parent or nested). GET_INFO just works with an input FD and an ID.
>>>>>> I'm not sure I get this, for nesting cases you can still make the
>>>>>> child an fd.
>>>>>>
>>>>>> And a question still, under what case we need to create multiple
>>>>>> ioasids on a single ioasid fd?
>>>>> One possible situation where multiple IOASIDs per FD could be used is
>>>>> that devices with different underlying IOMMU capabilities are sharing a
>>>>> single FD. In this case, only devices with consistent underlying IOMMU
>>>>> capabilities could be put in an IOASID and multiple IOASIDs per FD could
>>>>> be applied.
>>>>>
>>>>> Though, I still not sure about "multiple IOASID per-FD" vs "multiple
>>>>> IOASID FDs" for such case.
>>>> Right, that's exactly my question. The latter seems much more easier to
>>>> be understood and implemented.
>>>>
>>> A simple reason discussed in previous thread - there could be 1M's
>>> I/O address spaces per device while #FD's are precious resource.
>>
>> Is the concern for ulimit or performance? Note that we had
>>
>> #define NR_OPEN_MAX ~0U
>>
>> And with the fd semantic, you can do a lot of other stuffs: close on
>> exec, passing via SCM_RIGHTS.
> yes, fd has its merits.
>
>> For the case of 1M, I would like to know what's the use case for a
>> single process to handle 1M+ address spaces?
> This single process is Qemu with an assigned device. Within the guest
> there could be many guest processes. Though in reality I didn't see
> such 1M processes on a single device, better not restrict it in uAPI?


Sorry I don't get here.

We can open up to ~0U file descriptors, I don't see why we need to 
restrict it in uAPI.

Thanks


>
>>
>>> So this RFC treats fd as a container of address spaces which is each
>>> tagged by an IOASID.
>>
>> If the container and address space is 1:1 then the container seems useless.
>>
> yes, 1:1 then container is useless. But here it's assumed 1:M then
> even a single fd is sufficient for all intended usages.
>
> Thanks
> Kevin