On Fri, 8 Jun 2018 13:08:54 +1000 Alexey Kardashevskiy <a...@ozlabs.ru> wrote:
> On 8/6/18 8:15 am, Alex Williamson wrote: > > On Fri, 08 Jun 2018 07:54:02 +1000 > > Benjamin Herrenschmidt <b...@kernel.crashing.org> 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. vGPUs as implemented on M- and P-series Teslas aren't SR-IOV like either. That's why we have mdev devices now to implement software defined devices. I don't have first hand experience with V-series, but I would absolutely expect a PCIe-based Tesla V100 to support vGPU. > 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). I'm not gaining much confidence that we can rely on isolation between NVLink connected GPUs, it sounds like you're simply expecting that proprietary code from NVIDIA on a proprietary interconnect from NVIDIA is going to play nice and nobody will figure out how to do bad things because... obfuscation? Thanks, Alex
