Re: [Intel-gfx] [PATCH v5 4/4] drm/i915: add guard page to ggtt->error_capture

[Andrzej Hajda <andrzej.hajda@intel.com>]

On 09.03.2023 10:43, Tvrtko Ursulin wrote:
>
> On 09/03/2023 09:34, Andrzej Hajda wrote:
>>
>>
>> On 09.03.2023 10:08, Tvrtko Ursulin wrote:
>>>
>>> On 08/03/2023 15:39, Andrzej Hajda wrote:
>>>> Write-combining memory allows speculative reads by CPU.
>>>> ggtt->error_capture is WC mapped to CPU, so CPU/MMU can try
>>>> to prefetch memory beyond the error_capture, ie it tries
>>>> to read memory pointed by next PTE in GGTT.
>>>> If this PTE points to invalid address DMAR errors will occur.
>>>> This behaviour was observed on ADL and RPL platforms.
>>>> To avoid it, guard scratch page should be added after error_capture.
>>>> The patch fixes the most annoying issue with error capture but
>>>> since WC reads are used also in other places there is a risk similar
>>>> problem can affect them as well.
>>>>
>>>> v2:
>>>>    - modified commit message (I hope the diagnosis is correct),
>>>>    - added bug checks to ensure scratch is initialized on gen3 
>>>> platforms.
>>>>      CI produces strange stacktrace for it suggesting scratch[0] is 
>>>> NULL,
>>>>      to be removed after resolving the issue with gen3 platforms.
>>>> v3:
>>>>    - removed bug checks, replaced with gen check.
>>>> v4:
>>>>    - change code for scratch page insertion to support all platforms,
>>>>    - add info in commit message there could be more similar issues
>>>> v5:
>>>>    - check for nop_clear_range instead of gen8 (Tvrtko),
>>>>    - re-insert scratch pages on resume (Tvrtko)
>>>>
>>>> Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
>>>> Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
>>>> ---
>>>>   drivers/gpu/drm/i915/gt/intel_ggtt.c | 35 
>>>> +++++++++++++++++++++++++++++++----
>>>>   1 file changed, 31 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c 
>>>> b/drivers/gpu/drm/i915/gt/intel_ggtt.c
>>>> index b925da42c7cfc4..8fb700fde85c8f 100644
>>>> --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
>>>> +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
>>>> @@ -502,6 +502,21 @@ static void cleanup_init_ggtt(struct i915_ggtt 
>>>> *ggtt)
>>>>       mutex_destroy(&ggtt->error_mutex);
>>>>   }
>>>>   +static void
>>>> +ggtt_insert_scratch_pages(struct i915_ggtt *ggtt, u64 offset, u64 
>>>> length)
>>>> +{
>>>> +    struct i915_address_space *vm = &ggtt->vm;
>>>> +
>>>> +    if (vm->clear_range != nop_clear_range)
>>>
>>> Hm I thought usually we would add a prefix for exported stuff, like 
>>> in this case i915_vm_nop_clear_range, however I see intel_gtt.h 
>>> exports a bunch of stuff with no prefixes already so I guess you 
>>> could continue like that by inertia. The conundrum also could have 
>>> been avoided if you left it static (leaving out dpt and mock_gtt 
>>> patches) but no strong opinion from me.
>>>
>>>> +        return vm->clear_range(vm, offset, length);
>>>> +
>>>> +    while (length > 0) {
>>>> +        vm->insert_page(vm, px_dma(vm->scratch[0]), offset, 
>>>> I915_CACHE_NONE, 0);
>>>> +        offset += I915_GTT_PAGE_SIZE;
>>>> +        length -= I915_GTT_PAGE_SIZE;
>>>> +    }
>>>> +}
>>>> +
>>>>   static int init_ggtt(struct i915_ggtt *ggtt)
>>>>   {
>>>>       /*
>>>> @@ -550,8 +565,12 @@ static int init_ggtt(struct i915_ggtt *ggtt)
>>>>            * paths, and we trust that 0 will remain reserved. However,
>>>>            * the only likely reason for failure to insert is a driver
>>>>            * bug, which we expect to cause other failures...
>>>> +         *
>>>> +         * Since CPU can perform speculative reads on error capture
>>>> +         * (write-combining allows it) add scratch page after error
>>>> +         * capture to avoid DMAR errors.
>>>>            */
>>>> -        ggtt->error_capture.size = I915_GTT_PAGE_SIZE;
>>>> +        ggtt->error_capture.size = 2 * I915_GTT_PAGE_SIZE;
>>>>           ggtt->error_capture.color = I915_COLOR_UNEVICTABLE;
>>>>           if (drm_mm_reserve_node(&ggtt->vm.mm, &ggtt->error_capture))
>>>> drm_mm_insert_node_in_range(&ggtt->vm.mm,
>>>> @@ -561,11 +580,15 @@ static int init_ggtt(struct i915_ggtt *ggtt)
>>>>                               0, ggtt->mappable_end,
>>>>                               DRM_MM_INSERT_LOW);
>>>>       }
>>>> -    if (drm_mm_node_allocated(&ggtt->error_capture))
>>>> +    if (drm_mm_node_allocated(&ggtt->error_capture)) {
>>>> +        u64 start = ggtt->error_capture.start;
>>>> +        u64 size = ggtt->error_capture.size;
>>>> +
>>>> +        ggtt_insert_scratch_pages(ggtt, start, size);
>>>>           drm_dbg(&ggtt->vm.i915->drm,
>>>>               "Reserved GGTT:[%llx, %llx] for use by error capture\n",
>>>> -            ggtt->error_capture.start,
>>>> -            ggtt->error_capture.start + ggtt->error_capture.size);
>>>> +            start, start + size);
>>>> +    }
>>>>         /*
>>>>        * The upper portion of the GuC address space has a sizeable 
>>>> hole
>>>> @@ -1256,6 +1279,10 @@ void i915_ggtt_resume(struct i915_ggtt *ggtt)
>>>>         flush = i915_ggtt_resume_vm(&ggtt->vm);
>>>>   +    if (drm_mm_node_allocated(&ggtt->error_capture))
>>>> +        ggtt_insert_scratch_pages(ggtt, ggtt->error_capture.start,
>>>> +                      ggtt->error_capture.size);
>>>
>>> Maybe it belongs in i915_ggtt_resume_vm since that one deals with 
>>> PTEs? Looks like it to me, but ack either way.
>>
>> i915_ggtt_resume_vm is called for ggtt and dpt. Of course I could add 
>> conditionals there checking if it is ggtt, but in such situation 
>> i915_ggtt_resume seems more natural candidate.
>
> "if (drm_mm_node_allocated(&ggtt->error_capture))" check would handle 
> that automatically, no? i915_ggtt_resume has nothing about PTEs at the 
> moment..

Yes but since i915_ggtt_resume_vm  has vm as an argument (ie it operates 
on generic vm), there will be needed downcasting somewhere:
if (vm->is_ggtt) {
     struct i915_ggtt *ggtt = i915_vm_to_ggtt(vm);
     if (drm_mm_node_allocated(&ggtt->error_capture))
         ...
}

In i915_ggtt_resume we have it for free, but moreover 
i915_ggtt_resume_vm (despite its name) seems to handle common stuff of 
ggtt and dpt, and i915_ggtt_resume looks as specific for ggtt, similarly 
intel_dpt_resume is specific for dpt.
If it does not convince you, I will update patch with above code.

Regards
Andrzej

>
> Regards,
>
> Tvrtko