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

[Christian König]

Am 08.11.21 um 15:35 schrieb Felix Kuehling:
> 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.

Yeah, it's perfectly up to you if you want to risk that.

I've just double checked with running LTspice in wine and that indeed 
maps a lot of stuff starting at the 16KiB offset.

Not sure if that will ever collide with this, but I think that could 
indeed happen.

Regards,
Christian.

>
> Regards,
>    Felix
>
>
>> Regards,
>> Christian.
>>
>>> Signed-off-by: Alex Sierra <alex.sierra at 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)
>>>