linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Tue, 1 Jun 2021 14:07:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Liu Yi L <yi.l.liu@...ux.intel.com>
Cc:     "Alex Williamson (alex.williamson@...hat.com)\"\"" 
        <alex.williamson@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC] /dev/ioasid uAPI proposal


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.

For the case of 1M, I would like to know what's the use case for a 
single process to handle 1M+ address spaces?


> 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.

Thanks


>
> Thanks
> Kevin