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

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Hi Tvrtko,

On 2021/11/9 20:17, Tvrtko Ursulin wrote:
From: Tvrtko Ursulin<tvrtko.ursulin@xxxxxxxxx>

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@xxxxxxxxx>
Cc: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
---
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



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