Re: [PATCH v6 6/6] drm/i915/gvt: Adding interface so user space can get the dma-buf

From: Gerd Hoffmann <kraxel@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>,
	"Chen, Xiaoguang" <xiaoguang.chen@intel.com>
Cc: "intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Lv, Zhiyuan" <zhiyuan.lv@intel.com>,
	"intel-gvt-dev@lists.freedesktop.org"
	<intel-gvt-dev@lists.freedesktop.org>
Subject: Re: [PATCH v6 6/6] drm/i915/gvt: Adding interface so user space can get the dma-buf
Date: Fri, 02 Jun 2017 17:23:58 +0200	[thread overview]
Message-ID: <1496417038.16895.7.camel@redhat.com> (raw)
In-Reply-To: <20170602085831.41454a1f@t450s.home>

  Hi,

> > When i915's dma-buf's release() callback is called it will try to
> > free the gem object associated with the dma-buf if its ref count is
> > 0. But in our case the ref count is 1 so no free callback is called
> > so we can not release allocations there.

Why the ref count is one?  Who holds a reference and why?
Maybe it should be the other way around, i.e. the dmabuf holds a
reference on the vgpu instance backing it, i.e. you can't delete the
vgpu while dma-bufs exist?

> We cannot simply say that the user isn't allowed to release them in
> that order.

Yep, not going to fly.  Can happen even unintentionally because we can
pass around dmabufs to other processes.  Example: qemu passes dmabuf to
spice-client, then qemu crashes.  mgmt fd is closed before dmabuf fd
then.  The kernel must be able to handle that.

cheers,
  Gerd
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