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

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

On 2021/11/10 17:30, Tvrtko Ursulin wrote:
> 
> On 10/11/2021 07:12, Lu Baolu wrote:
>> 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.
> 
> Yes it seems to work as is:
> 
> default:
> 
> # grep -i iommu /sys/kernel/debug/dri/*/i915_capabilities
> /sys/kernel/debug/dri/0/i915_capabilities:iommu: enabled
> /sys/kernel/debug/dri/1/i915_capabilities:iommu: enabled
> 
> intel_iommu=igfx_off:
> 
> # grep -i iommu /sys/kernel/debug/dri/*/i915_capabilities
> /sys/kernel/debug/dri/0/i915_capabilities:iommu: disabled
> /sys/kernel/debug/dri/1/i915_capabilities:iommu: enabled
> 
> On my system dri device 0 is integrated graphics and 1 is discrete.

The drm device 0 has a dedicated iommu. When the user request igfx not
mapped, the VT-d implementation will turn it off to save power. But for
shared iommu, you definitely will get it enabled.

Best regards,
baolu