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

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

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.

Regards
Andrzej

>
> Regards,
>
> Tvrtko
>
>> +
>>       ggtt->invalidate(ggtt);
>>         if (flush)
>>