On 8/6/18 8:15 am, Alex Williamson wrote: > On Fri, 08 Jun 2018 07:54:02 +1000 > Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote: > >> On Thu, 2018-06-07 at 11:04 -0600, Alex Williamson wrote: >>> >>> Can we back up and discuss whether the IOMMU grouping of NVLink >>> connected devices makes sense? AIUI we have a PCI view of these >>> devices and from that perspective they're isolated. That's the view of >>> the device used to generate the grouping. However, not visible to us, >>> these devices are interconnected via NVLink. What isolation properties >>> does NVLink provide given that its entire purpose for existing seems to >>> be to provide a high performance link for p2p between devices? >> >> Not entire. On POWER chips, we also have an nvlink between the device >> and the CPU which is running significantly faster than PCIe. >> >> But yes, there are cross-links and those should probably be accounted >> for in the grouping. > > Then after we fix the grouping, can we just let the host driver manage > this coherent memory range and expose vGPUs to guests? The use case of > assigning all 6 GPUs to one VM seems pretty limited. (Might need to > convince NVIDIA to support more than a single vGPU per VM though) These are physical GPUs, not virtual sriov-alike things they are implementing as well elsewhere. My current understanding is that every P9 chip in that box has some NVLink2 logic on it so each P9 is directly connected to 3 GPUs via PCIe and 2xNVLink2, and GPUs in that big group are interconnected by NVLink2 links as well. >From small bits of information I have it seems that a GPU can perfectly work alone and if the NVIDIA driver does not see these interconnects (because we do not pass the rest of the big 3xGPU group to this guest), it continues with a single GPU. There is an "nvidia-smi -r" big reset hammer which simply refuses to work until all 3 GPUs are passed so there is some distinction between passing 1 or 3 GPUs, and I am trying (as we speak) to get a confirmation from NVIDIA that it is ok to pass just a single GPU. So we will either have 6 groups (one per GPU) or 2 groups (one per interconnected group). -- Alexey
