On Thu, Feb 07, 2019 at 08:26:20PM -0700, Alex Williamson wrote: > On Fri, 8 Feb 2019 13:29:37 +1100 > Alexey Kardashevskiy wrote: > > > On 08/02/2019 02:18, Alex Williamson wrote: > > > On Thu, 7 Feb 2019 15:43:18 +1100 > > > Alexey Kardashevskiy wrote: > > > > > >> On 07/02/2019 04:22, Daniel Henrique Barboza wrote: > > >>> Based on this series, I've sent a Libvirt patch to allow a QEMU process > > >>> to inherit IPC_LOCK when using VFIO passthrough with the Tesla V100 > > >>> GPU: > > >>> > > >>> https://www.redhat.com/archives/libvir-list/2019-February/msg00219.html > > >>> > > >>> > > >>> In that thread, Alex raised concerns about allowing QEMU to freely lock > > >>> all the memory it wants. Is this an issue to be considered in the review > > >>> of this series here? > > >>> > > >>> Reading the patches, specially patch 3/3, it seems to me that QEMU is > > >>> going to lock the KVM memory to populate the NUMA node with memory > > >>> of the GPU itself, so at first there is no risk of not taking over the > > >>> host RAM. > > >>> Am I missing something? > > >> > > >> > > >> The GPU memory belongs to the device and not visible to the host as > > >> memory blocks and not covered by page structs, for the host it is more > > >> like MMIO which is passed through to the guest without that locked > > >> accounting, I'd expect libvirt to keep working as usual except that: > > >> > > >> when libvirt calculates the amount of memory needed for TCE tables > > >> (which is guestRAM/64k*8), now it needs to use the end of the last GPU > > >> RAM window as a guest RAM size. For example, in QEMU HMP "info mtree -f": > > >> > > >> FlatView #2 > > >> AS "memory", root: system > > >> AS "cpu-memory-0", root: system > > >> Root memory region: system > > >> 0000000000000000-000000007fffffff (prio 0, ram): ppc_spapr.ram > > >> 0000010000000000-0000011fffffffff (prio 0, ram): nvlink2-mr > > >> > > >> So previously the DMA window would cover 0x7fffffff+1, now it has to > > >> cover 0x11fffffffff+1. > > > > > > This looks like a chicken and egg problem, you're saying libvirt needs > > > to query mtree to understand the extent of the GPU layout, but we need > > > to specify the locked memory limits in order for QEMU to start? Is > > > libvirt supposed to start the VM with unlimited locked memory and fix > > > it at some indeterminate point in the future? Run a dummy VM with > > > unlimited locked memory in order to determine the limits for the real > > > VM? Neither of these sound practical. Thanks, > > > > > > QEMU maps GPU RAM at known locations (which only depends on the vPHB's > > index or can be set explicitely) and libvirt knows how many GPUs are > > passed so it is quite easy to calculate the required amount of memory. > > > > Here is the window start calculation: > > https://github.com/aik/qemu/commit/7073cad3ae7708d657e01672bcf53092808b54fb#diff-662409c2a5a150fe231d07ea8384b920R3812 > > > > We do not exactly know the GPU RAM window size until QEMU reads it from > > VFIO/nvlink2 but we know that all existing hardware has a window of > > 128GB (the adapters I have access to only have 16/32GB on board). > > So you're asking that libvirt add 128GB per GPU with magic nvlink > properties, which may be 8x what's actually necessary and libvirt > determines which GPUs to apply this to how? Does libvirt need to sort > through device tree properties for this? Thanks, Hm. If the GPU memory is really separate from main RAM, which it sounds like, I don't think it makes sense to account it against the same locked memory limit as regular RAM. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson