[Mesa-dev] [PATCH] radv: fix possible GPU hangs by zeroing the descriptor set

[Samuel Pitoiset]

On 09/08/2017 04:08 PM, Józef Kucia wrote:
> On Fri, Sep 8, 2017 at 2:43 PM, Samuel Pitoiset
> <samuel.pitoiset at gmail.com> wrote:
>> when an application doesn't update the contents of a descriptor
>> set object, it contains random values and this might end up with
>> GPU VM faults if a shader tries to access that descriptor.
>>
>> The Vulkan spec doesn't state what a driver should do when
>> a descriptor is unset, but zeroing the contents seems to be
>> a safe solution.
> 
> FWIW, this is invalid usage. The Vulkan spec states: "Descriptors in
> each bound descriptor set, specified via vkCmdBindDescriptorSets, must
> be
> valid if they are statically used by the currently bound VkPipeline
> object, specified via
> vkCmdBindPipeline".

Thanks for this, but it appears that some drivers do the same thing (or 
use any other default values). I think the real question here is: does 
this affect performance? If so, we should avoid it.

The application probably needs a fix too, but this has the advantage to 
prevent future similar hangs.

Opinions welcome!

> 
>>
>> This fixes a GPU hang with Unity because a pixel shader tries
>> to access a descriptor that is unset by the application.
>> Though, maybe that descriptor load should have been DCE'd.
>>
>> Cc: Pierre-Loup A. Griffais <pgriffais at valvesoftware.com>
>> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
>> ---
>>   src/amd/vulkan/radv_descriptor_set.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/src/amd/vulkan/radv_descriptor_set.c b/src/amd/vulkan/radv_descriptor_set.c
>> index 260bfbc12b..6271f9d06e 100644
>> --- a/src/amd/vulkan/radv_descriptor_set.c
>> +++ b/src/amd/vulkan/radv_descriptor_set.c
>> @@ -327,6 +327,12 @@ radv_descriptor_set_create(struct radv_device *device,
>>                          list_add(&set->vram_list, prev);
>>                  } else
>>                          return vk_error(VK_ERROR_OUT_OF_POOL_MEMORY_KHR);
>> +
>> +               /* Initialize the descriptor set to zero in order to avoid GPU
>> +                * VM faults when an application doesn't update the contents of
>> +                * a descriptor set object.
>> +                */
>> +               memset(set->mapped_ptr, 0, layout->size);
>>          }
>>
>>          for (unsigned i = 0; i < layout->binding_count; ++i) {
>> --
>> 2.14.1
>>
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