Support for amdgpu VM update via CPU on large-bar systems

zhoucm1 david1.zhou at
Fri May 12 08:37:39 UTC 2017

On 2017年05月12日 16:33, Christian König wrote:
> Am 12.05.2017 um 10:25 schrieb zhoucm1:
>> On 2017年05月10日 05:47, Kasiviswanathan, Harish wrote:
>>> Hi,
>>> Please review the patch set that supports amdgpu VM update via CPU. This feature provides improved performance for compute (HSA) where mapping / unmapping is carried out (by Kernel) independent of command submissions (done directly by user space). This version doesn't support shadow copy of VM page tables for CPU based update.
>> I think your improved performance is from less waiting for cs, 
>> generally, SDMA engine updating page table is faster than CPU, 
>> otherwise we don't need sdma for updating PT.
>> So whether your this improvement proves we have some redundant sync 
>> when mapping / unmapping, if yes, we should fix that, then not sure 
>> if CPU method is need or not.
> The problem is that the KFD is designed synchronously for page table 
> updates. In other words they need to wait for the update to finish and 
> that takes time.
> Apart from that your comment is absolutely correct, we found that the 
> SDMA is sometimes much faster to do the update than the CPU.
If the sdma is faster, even they wait for finish, which time is shorter 
than CPU, isn't it? Of course, the precondition is sdma is exclusive. 
They can reserve a sdma for PT updating.

David Zhou
> Regards,
> Christian.
>> Regards,
>> David Zhou
>>> Best Regards,
>>> Harish
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