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

Fri Nov 12 00:53:10 UTC 2021

On 11/11/21 11:06 PM, Tvrtko Ursulin wrote:
> 
> On 10/11/2021 12:35, Lu Baolu wrote:
>> On 2021/11/10 20:08, Tvrtko Ursulin wrote:
>>>
>>> On 10/11/2021 12:04, Lu Baolu wrote:
>>>> 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 at 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 at intel.com>
>>>>>>> Cc: Lu Baolu<baolu.lu at 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.
>>>
>>> Sorry I am not following, what exactly do you mean? Is there a 
>>> platform with integrated graphics without a dedicated iommu, in which 
>>> case intel_iommu=igfx_off results in intel_iommu_gfx_mapped == 0 and 
>>> iommu_get_domain_for_dev returning non-NULL?
>>
>> Your code always work for an igfx with a dedicated iommu. This might be
>> always true on today's platforms. But from driver's point of view, we
>> should not make such assumption.
>>
>> For example, if the iommu implementation decides not to turn off the
>> graphic iommu (perhaps due to some hw quirk or for graphic
>> virtualization), your code will be broken.
> 
> I tried your suggestion (checking for __IOMMU_DOMAIN_PAGING) and it 
> works better, however I have observed one odd behaviour (for me at least).
> 
> In short - why does the DMAR mode for the discrete device change 
> depending on igfx_off parameter?
> 
> Consider the laptop has these two graphics cards:
> 
> # cat /sys/kernel/debug/dri/0/name
> i915 dev=0000:00:02.0 unique=0000:00:02.0 # integrated
> 
> # cat /sys/kernel/debug/dri/1/name
> i915 dev=0000:03:00.0 unique=0000:03:00.0 # discrete
> 
> Booting with different options:
> ===============================
> 
> default / intel_iommu=on
> ------------------------
> 
> # cat /sys/class/iommu/dmar0/devices/0000:00:02.0/iommu_group/type
> DMA-FQ
> # cat /sys/class/iommu/dmar2/devices/0000:03:00.0/iommu_group/type
> DMA-FQ
> 
> # 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
> 
> All good.
> 
> intel_iommu=igfx_off
> --------------------
> 
> ## no dmar0 in sysfs
> # cat /sys/class/iommu/dmar2/devices/0000:03:00.0/iommu_group/type
> identity
> 
> Unexpected!?
> 
> # 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: disabled # At least the 
> i915 patch detects it correctly.
> 
> intel_iommu=off
> ---------------
> 
> ## no dmar0 in sysfs
> ## no dmar2 in sysfs
> 
> # 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: disabled
> 
> All good.
> 
> The fact discrete graphics changes from translated to pass-through when 
> igfx_off is set is surprising to me. Is this a bug?

The existing VT-d implementation doesn't distinguish igfx from dgfx. It
only checks whether the device is of a display class:

#define IS_GFX_DEVICE(pdev) ((pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY)

When igfx_off is specified, all graphic devices will put into pass-
through (the same meaning as identity mapping) mode. For igfx, since
the iommu is always dedicated, hence it further turn off the iommu
(hence there's no iommu domain) to save power.

Best regards,
baolu