[PATCH v1 1/2] drm/i915: Introduce GEM proxy

[Zhang, Tina]

> -----Original Message-----
> From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> Sent: Tuesday, October 17, 2017 10:37 PM
> To: Zhang, Tina <tina.zhang at intel.com>; zhenyuw at linux.intel.com; Lv, Zhiyuan
> <zhiyuan.lv at intel.com>; Wang, Zhi A <zhi.a.wang at intel.com>; Tian, Kevin
> <kevin.tian at intel.com>; daniel at ffwll.ch
> Cc: Zhang, Tina <tina.zhang at intel.com>; intel-gfx at lists.freedesktop.org; intel-
> gvt-dev at lists.freedesktop.org; Daniel Vetter <daniel.vetter at ffwll.ch>
> Subject: Re: [PATCH v1 1/2] drm/i915: Introduce GEM proxy
> 
> Quoting Tina Zhang (2017-10-16 09:57:33)
> > GEM proxy is a kind of GEM whose backing physical memory is pinned and
> > produced by guest VM and is used by host as read only. With GEM proxy,
> > host is able to access guest physical memory through GEM object
> > interface. As GEM proxy is such a special kind of GEM, a new flag
> > I915_GEM_OBJECT_IS_PROXY is introduced to ban host from changing the
> > backing storage of GEM proxy.
> >
> > V1:
> > - the patch is separated from the "Dma-buf support for Gvt-g"
> >   patch-set. (Joonas)
> >
> > Here are the histories of this patch in "Dma-buf support for Gvt-g"
> > patch-set:
> >
> > v14:
> > - return -ENXIO when gem proxy object is banned by ioctl.
> >   (Chris) (Daniel)
> >
> > v13:
> > - add comments to GEM proxy. (Chris)
> > - don't ban GEM proxy in i915_gem_sw_finish_ioctl. (Chris)
> > - check GEM proxy bar after finishing i915_gem_object_wait. (Chris)
> > - remove GEM proxy bar in i915_gem_madvise_ioctl.
> >
> > v6:
> > - add gem proxy barrier in the following ioctls. (Chris)
> >   i915_gem_set_caching_ioctl
> >   i915_gem_set_domain_ioctl
> >   i915_gem_sw_finish_ioctl
> >   i915_gem_set_tiling_ioctl
> >   i915_gem_madvise_ioctl
> 
> gem_busy: sees the local object, not the proxy activity; that's ok
> get_caching: returns the constant value
> pread: works by forcing GTT access
> pwrite: works by forcing GTT access
> mmap: disallowed by !filp, errno fixup in next patch
> mmap_gtt: works
> set-domain: fixed
> sw-finish: noted as impossible (please add checks)
I thought we were agreed on this: https://lists.freedesktop.org/archives/intel-gvt-dev/2017-July/001523.html
So, in this version I just add the comments without checking

> set-tiling: fixed
> get_tiling: returns constant value
> madvise: works, since it's a discard of the local page array overlay-put-image?
No. The original idea is that the GEM proxy could support madvise as it only invokes unpin/pin. And supporting madivse might benefit userspace.
So, do you think we'd better ban GEM proxy in madvise?

> not supported on any relevant platforms!
> gem_wait: works on local bo (as expected)
> 
> execbuf: works (or should at least work fine on a proxy object as either a batch
> or one of the auxiliaries)
> 
> Hmm, use as a batch + cmdparser is broken; should result in ENODEV to
> userspace. An admittedly odd result, but not an oops.
GEM proxy isn't designed to be used as a batch buffer, only as an auxiliary. So I think here you mean we need to ban GEM proxy in i915_gem_execbuffer/i915_gem_execbuffer2, right?

> 
> display: pin-to-display and fencing will work, I think, and flush_for_display is
> irrelevant as no direct cpu access.
> 
> dmabuf: works partially, but what doesn't work is optional. Userspace mmap is
> barred, most inter-driver interaction is barred. On paper a nuisance should it
> want to be used, but we should not oops. Hmm, no, we need to make the
> GEM_BUG_ON(!struct_page(obj)) in
> i915_gem_object_pin_map() a real return.
So, as this can benefit all kinds of gem object w/o backing storage, can we split the following part into another patch?
Thanks.

BR,
Tina
> 
> Please add
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c
> b/drivers/gpu/drm/i915/i915_gem.c index d699ea3ab80b..6a7ca9a502ba
> 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2650,7 +2650,8 @@ void *i915_gem_object_pin_map(struct
> drm_i915_gem_object *obj,
>         void *ptr;
>         int ret;
> 
> -       GEM_BUG_ON(!i915_gem_object_has_struct_page(obj));
> +       if (unlikely(!i915_gem_object_has_struct_page(obj)))
> +               return ERR_PTR(-ENODEV);
> 
>         ret = mutex_lock_interruptible(&obj->mm.lock);
>         if (ret)
> 
> and my
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk> -Chris