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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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..

Regards,

Tvrtko