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

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Date:   Tue, 02 May 2017 11:50:58 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Xiaoguang Chen <xiaoguang.chen@...el.com>
Cc:     alex.williamson@...hat.com, intel-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org, zhi.a.wang@...el.com,
        zhenyuw@...ux.intel.com, linux-kernel@...r.kernel.org,
        zhiyuan.lv@...el.com, kevin.tian@...el.com
Subject: Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

On Fr, 2017-04-28 at 17:35 +0800, Xiaoguang Chen wrote:
> +static size_t intel_vgpu_reg_rw_gvtg(struct intel_vgpu *vgpu, char
> *buf,
> +               size_t count, loff_t *ppos, bool iswrite)
> +{
> +       unsigned int i = VFIO_PCI_OFFSET_TO_INDEX(*ppos) -
> +                       VFIO_PCI_NUM_REGIONS;
> +       loff_t pos = *ppos & VFIO_PCI_OFFSET_MASK;
> +       int fd;
> +
> +       if (pos >= vgpu->vdev.region[i].size || iswrite) {
> +               gvt_vgpu_err("invalid op or offset for Intel vgpu fd
> region\n");
> +               return -EINVAL;
> +       }
> +
> +       fd = anon_inode_getfd("gvtg", &intel_vgpu_gvtg_ops, vgpu,
> +                       O_RDWR | O_CLOEXEC);
> +       if (fd < 0) {
> +               gvt_vgpu_err("create intel vgpu fd failed:%d\n", fd);
> +               return -EINVAL;
> +       }
> +
> +       count = min(count, (size_t)(vgpu->vdev.region[i].size - pos));
> +       memcpy(buf, &fd, count);
> +
> +       return count;
> +}

Hmm, that looks like a rather strange way to return a file descriptor.

What is the reason to not use ioctls on the vfio file handle, like older
version of these patches did?

cheers,
  Gerd