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Subject: Re: [RFC] /dev/ioasid uAPI proposal
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Liu Yi L <yi.l.liu@linux.intel.com>,
        "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>, David Woodhouse <dwmw2@infradead.org>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <c84787ec-9d8f-3198-e800-fe0dc8eb53c7@redhat.com>
Date:   Tue, 8 Jun 2021 09:10:42 +0800
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在 2021/6/3 上午1:21, Jason Gunthorpe 写道:
> On Wed, Jun 02, 2021 at 04:54:26PM +0800, Jason Wang wrote:
>> 在 2021/6/2 上午1:31, Jason Gunthorpe 写道:
>>> On Tue, Jun 01, 2021 at 04:47:15PM +0800, Jason Wang wrote:
>>>> We can open up to ~0U file descriptors, I don't see why we need to restrict
>>>> it in uAPI.
>>> There are significant problems with such large file descriptor
>>> tables. High FD numbers man things like select don't work at all
>>> anymore and IIRC there are more complications.
>>
>> I don't see how much difference for IOASID and other type of fds. People can
>> choose to use poll or epoll.
> Not really, once one thing in an applicate uses a large number FDs the
> entire application is effected. If any open() can return 'very big
> number' then nothing in the process is allowed to ever use select.
>
> It is not a trivial thing to ask for
>
>> And with the current proposal, (assuming there's a N:1 ioasid to ioasid). I
>> wonder how select can work for the specific ioasid.
> pagefault events are one thing that comes to mind. Bundling them all
> together into a single ring buffer is going to be necessary. Multifds
> just complicate this too
>
> Jason


Well, this sounds like a re-invention of io_uring which has already 
worked for multifds.

Thanks