linux-kernel - Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

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Date:   Fri, 19 May 2017 10:04:49 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     "Chen, Xiaoguang" <xiaoguang.chen@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>,
        "Lv, Zhiyuan" <zhiyuan.lv@...el.com>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "Wang, Zhi A" <zhi.a.wang@...el.com>
Subject: Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

  Hi,

> > User space need to check whether there's a dmabuf for the plane(user space usually cached two or three dmabuf to handle double buffer or triple buffer situation) only there's no dmabuf for the plane we will create a dmabuf for it(another ioctl).
> 
> If our ioctls are "Query current plane" and "Give me a dmabuf for
> current plane", isn't that racey?  The current plane could have changed
> between those two calls so the user doesn't absolutely know which plane
> the dmabuf retrieved is for.  The "Give me a dmabuf" therefore needs to
> take some sort of plane index so the user can request a specific
> plane.

The "give me a dmabuf" ioctl returns the plane description too, so
userspace can at least figure it did hit the race window.

We could also do it the other way around:  Instead of having the kernel
returning the plane description userspace could pass it in, and the
kernel throws -EINVAL in case it doesn't match due to things having
changed meanwhile.

cheers,
  Gerd