On Mon, Sep 29, 2014 at 07:44:56PM +0800, Jike Song wrote:
> On 09/19/2014 03:25 PM, Chris Wilson wrote:
> >Now, given that these are simply trapped memory access, wouldn't it be
> >simply to have:
> >
> >struct i915_virtual_gpu {
> > struct vgt_if *if;
> >} vgu;
> >
> >static inline bool intel_vgpu_active(struct drm_i915_private *i915) { return
> >i915->vgpu.if; }
> >
> >then you have constructs like
> >void i915_check_vgpu(struct drm_i915_private *i915)
> >{
> > struct vgt_if *if;
> >
> > if = i915->regs + VGT_PVINFO_PAGE;
> > if (if->magic != VGT_MAGIC)
> > return;
> >
> > if (INTEL_VGT_IF_VERSION !=
> > INTEL_VGT_IF_VERSION_ENCODE(if->version_major,
> > if->version_minor))
> > return;
> >
> >
> > i915->vgpu.if = if;
> >}
> >
> >And later, for example:
> >
> >if (intel_vgpu_active(dev_priv))
> > dev_priv->num_fence_regs = dev_priv->vgpu.if->fence_num;
> >
>
> Hi Chris, sorry that I didn't understand you correctly. After discussion
> with Yu today, I realized that unfortunately, the vgt_if can't be
> dereferenced directly.
>
> There are several reasons:
>
> - dereferencing a MMIO address will be complained by sparse(1)
>
> - for Guest VM, such accesses will be trapped by hypervisor, and
> hence emulated correctly; However this doesn't work for Host(e.g.
> Domain 0 of Xen, the Linux host KVM resides in). For host, we used
> a proactive mechanism to redirect i915 MMIO accesses to vgt,
> the GPU device-model, for the sake of central management & sharing
> among VMs, including host.
You only need to be careful during vgpu detection. After that you know
everything is safe. If you do the detection during intel_uncore_init(),
or similar, you can use raw mmio access and explict sparse annotations
to do the right thing.
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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