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

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Message-ID: <20210602172154.GC1002214@nvidia.com>
Date:   Wed, 2 Jun 2021 14:21:54 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Liu Yi L <yi.l.liu@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Alex Williamson (alex.williamson@...hat.com)\"\"" 
        <alex.williamson@...hat.com>, David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

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