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Date:   Tue, 9 Aug 2022 08:23:27 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Oded Gabbay <oded.gabbay@gmail.com>,
        Dave Airlie <airlied@gmail.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Yuji Ishikawa <yuji2.ishikawa@toshiba.co.jp>,
        Jiho Chu <jiho.chu@samsung.com>, Arnd Bergmann <arnd@arndb.de>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>
Subject: Re: New subsystem for acceleration devices
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On Mon, Aug 08, 2022 at 02:55:28PM -0300, Jason Gunthorpe wrote:
> On Mon, Aug 08, 2022 at 08:10:22AM +0200, Greg Kroah-Hartman wrote:
> > On Sun, Aug 07, 2022 at 02:25:33PM +0300, Oded Gabbay wrote:
> > > 2. Common code to handle drivers that want to allow a single user at a
> > > time to run open the device char file.
> > 
> > Note, that's an impossible request, and one that the kernel should never
> > worry about, so don't even try it.  Think about userspace doing an call
> > to dup() on an open char file descriptor and then passing that off
> > somewhere else.
> 
> Oded is talking about a model like VFIO has where the HW has a limited
> number of concurrent state registers - lets say in this case the ASID
> translation mapping the accelerator into DMA.

Based on the number of drivers that I see submitted weekly that try to
restrict their open call to just one user by using atomic variables or
other tricks, I think my interpretation of this stands :)

> Each 'struct file' that is created owns one of those HW state
> registers, and each struct file is completely isolated from all
> others. eg someone controlling the accelerator through struct file A
> cannot access memory mapped into the accelerator through struct file
> B.
> 
> So, the number of struct files that can be created is capped at the
> number of HW state registers the device can support (eg one for
> Habana).
> 
> This is different from the number of FDs pointing at the struct file.
> Userpsace can open a HW state and point a lot of FDs at it, that is
> userspace's problem. From a kernel view they all share one struct file
> and thus one HW state.

Yes, that's fine, if that is what is happening here, I have no
objection.

greg k-h