Re: [PATCH 1/8] drm/i915: Introduce a PV INFO page structure for Intel GVT-g.

[Jike Song <jike.song@intel.com>]

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.

Given that, though technically your code works for Guest, but after the
integration of host support of iGVT, we still need to use I915_READ/I915_WRITE
then. The host patches is soon to posted for your review :)

I should have realized that earlier, sorry!


> -Chris
>

--
Thanks,
Jike