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

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Message-ID: <20170504100833.199bc8ba@t450s.home>
Date:   Thu, 4 May 2017 10:08:33 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Chen, Xiaoguang" <xiaoguang.chen@...el.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "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>,
        Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

On Thu, 4 May 2017 03:09:40 +0000
"Chen, Xiaoguang" <xiaoguang.chen@...el.com> wrote:

> Hi Alex, do you have any comments for this interface?
> 
> >-----Original Message-----
> >From: intel-gvt-dev [mailto:intel-gvt-dev-bounces@...ts.freedesktop.org] On
> >Behalf Of Chen, Xiaoguang
> >Sent: Wednesday, May 03, 2017 9:39 AM
> >To: Gerd Hoffmann <kraxel@...hat.com>
> >Cc: Tian, Kevin <kevin.tian@...el.com>; intel-gfx@...ts.freedesktop.org; linux-
> >kernel@...r.kernel.org; zhenyuw@...ux.intel.com; alex.williamson@...hat.com;
> >Lv, Zhiyuan <zhiyuan.lv@...el.com>; 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
> >
> >
> >  
> >>-----Original Message-----
> >>From: Gerd Hoffmann [mailto:kraxel@...hat.com]
> >>Sent: Tuesday, May 02, 2017 5:51 PM
> >>To: Chen, Xiaoguang <xiaoguang.chen@...el.com>
> >>Cc: alex.williamson@...hat.com; intel-gfx@...ts.freedesktop.org;
> >>intel-gvt- dev@...ts.freedesktop.org; Wang, Zhi A
> >><zhi.a.wang@...el.com>; zhenyuw@...ux.intel.com;
> >>linux-kernel@...r.kernel.org; Lv, Zhiyuan <zhiyuan.lv@...el.com>; Tian,
> >>Kevin <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?  
> >If I understood correctly that Alex prefer not to change the ioctls on the vfio file
> >handle like the old version.
> >So I used this way the smallest change to general vfio framework only adding a
> >subregion definition.

I think I was hoping we could avoid a separate file descriptor
altogether and use a vfio region instead.  However, it was explained
previously why this really needs to be a separate fd and I agree that
using a region to expose an fd is really awkward.  If we're going to
have a separate fd, let's use a device specific ioctl to get it.
Thanks,

Alex