Re: [Intel-gfx] [PATCH] drm/i915: Use per device iommu check

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi Tvrtko,

On 2021/11/9 20:17, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin<tvrtko.ursulin@intel.com>
> 
> On igfx + dgfx setups, it appears that intel_iommu=igfx_off option only
> disables the igfx iommu. Stop relying on global intel_iommu_gfx_mapped
> and probe presence of iommu domain per device to accurately reflect its
> status.
> 
> Signed-off-by: Tvrtko Ursulin<tvrtko.ursulin@intel.com>
> Cc: Lu Baolu<baolu.lu@linux.intel.com>
> ---
> Baolu, is my understanding here correct? Maybe I am confused by both
> intel_iommu_gfx_mapped and dmar_map_gfx being globals in the intel_iommu
> driver. But it certainly appears the setup can assign some iommu ops (and
> assign the discrete i915 to iommu group) when those two are set to off.

diff --git a/drivers/gpu/drm/i915/i915_drv.h 
b/drivers/gpu/drm/i915/i915_drv.h
index e967cd08f23e..9fb38a54f1fe 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1763,26 +1763,27 @@ static inline bool run_as_guest(void)
  #define HAS_D12_PLANE_MINIMIZATION(dev_priv) (IS_ROCKETLAKE(dev_priv) || \
  					      IS_ALDERLAKE_S(dev_priv))

-static inline bool intel_vtd_active(void)
+static inline bool intel_vtd_active(struct drm_i915_private *i915)
  {
-#ifdef CONFIG_INTEL_IOMMU
-	if (intel_iommu_gfx_mapped)
+	if (iommu_get_domain_for_dev(i915->drm.dev))
  		return true;
-#endif

  	/* Running as a guest, we assume the host is enforcing VT'd */
  	return run_as_guest();
  }

Have you verified this change? I am afraid that
iommu_get_domain_for_dev() always gets a valid iommu domain even
intel_iommu_gfx_mapped == 0.

A possible way could look like this:

static bool intel_vtd_active(struct drm_i915_private *i915)
{
         struct iommu_domain *domain;

         domain = iommu_get_domain_for_dev(i915->drm.dev);

         if (domain && (domain->type & __IOMMU_DOMAIN_PAGING))
                 return true;

	... ...
}

Actually I don't like this either since it checks the domain->type out
of the iommu subsystem. We could refactor this later by export an iommu
interface for this check.

Best regards,
baolu