Re: [PATCH] drm/amdkfd: lower the VAs base offset to 8KB

[Felix Kuehling <felix.kuehling@amd.com>]

Am 2021-11-08 um 5:22 a.m. schrieb Christian König:
> Am 05.11.21 um 20:25 schrieb Alex Sierra:
>> The low 16MB of virtual address space are currently reserved for kernel
>> mode allocations mapped into user virtual address space. This causes
>> conflicts with HMM/SVM mappings at low virtual addresses. We tried to
>> move those kernel mode allocations to the upper half of the 64-bit
>> virtual address space for GFX9, which is naturally reserved for kernel
>> use. However, TBA (trap handler code) has problems to access addresses
>> in the high virtual space. We have decided to set this to 8KB of the
>> lower address space as a temporary fix, while investigate TBA address
>> problem. It is very unlikely for user space to map memory at this low
>> region.
>
> Unfortunately that is not correct. IIRC every program run inside wine
> maps something at the 16-64KiB addresses.
>
> Don't ask me what that stuff is good for, but we already ran into
> problems because of exactly that.
It's only a problem if the application tries to map that memory on the
GPU through the HMM/SVM API. As long as there is no conflict with GPU
mappings created by the application, the GPU can safely use those
addresses for its purposes.

Regards,
  Felix


>
> Regards,
> Christian.
>
>>
>> Signed-off-by: Alex Sierra <alex.sierra@amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdkfd/kfd_flat_memory.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_flat_memory.c
>> b/drivers/gpu/drm/amd/amdkfd/kfd_flat_memory.c
>> index 2e86692def19..d1388896f9c1 100644
>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_flat_memory.c
>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_flat_memory.c
>> @@ -308,7 +308,7 @@
>>    * 16MB are reserved for kernel use (CWSR trap handler and kernel IB
>>    * for now).
>>    */
>> -#define SVM_USER_BASE 0x1000000ull
>> +#define SVM_USER_BASE (u64)(KFD_CWSR_TBA_TMA_SIZE + 2*PAGE_SIZE)
>>   #define SVM_CWSR_BASE (SVM_USER_BASE - KFD_CWSR_TBA_TMA_SIZE)
>>   #define SVM_IB_BASE   (SVM_CWSR_BASE - PAGE_SIZE)
>>   
>