linux-kernel - Re: [Intel-gfx] [PATCH v9 5/7] vfio: Define vfio based dma-buf operations

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Date:   Fri, 23 Jun 2017 10:40:43 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Zhi Wang <zhi.a.wang@...el.com>,
        "Wang, Zhenyu Z" <zhenyu.z.wang@...el.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Chen, Xiaoguang" <xiaoguang.chen@...el.com>,
        "Zhang, Tina" <tina.zhang@...el.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Lv, Zhiyuan" <zhiyuan.lv@...el.com>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>
Subject: Re: [Intel-gfx] [PATCH v9 5/7] vfio: Define vfio based dma-buf
 operations

On Fri, 23 Jun 2017 10:31:28 +0200
Gerd Hoffmann <kraxel@...hat.com> wrote:

> On Fri, 2017-06-23 at 15:49 +0800, Zhi Wang wrote:
> > Hi:
> >      Thanks for the discussions! If the userspace application has 
> > already maintained a LRU list, it looks like we don't need
> > generation 
> > anymore,  
> 
> generation isn't required, things are working just fine without that. 
> It is just a small optimization, userspace can skip the LRU lookup
> altogether if the generation didn't change.
> 
> But of couse that only pays off if the kernel doesn't has to put much
> effort into maintaining the generation id.  Something simple like
> increasing it each time the guest writes a register which affects
> plane_info.

But it seems like that simple management algorithm pretty much
guarantees that the kernel will never revisit a generation and
therefore caching dmabuf fds is pointless.  AIUI the optimization is to
allow userspace to 'at a glance' test one plane_info vs another.  The
user could also do this with a memcmp of the plane_info structs if
that's its only purpose.  A randomly incremented field within that
struct could actually be a hindrance to the user for such a
comparison.  Are there cases where the plane_info struct is otherwise
identical where the user would need to get a new dmabuf fd anyway?
Thanks,

Alex