On Mon, Feb 11, 2019 at 02:49:49PM +1100, Alexey Kardashevskiy wrote: > > > On 08/02/2019 16:28, David Gibson wrote: > > On Thu, Feb 07, 2019 at 08:26:20PM -0700, Alex Williamson wrote: > >> On Fri, 8 Feb 2019 13:29:37 +1100 > >> Alexey Kardashevskiy <a...@ozlabs.ru> wrote: > >> > >>> On 08/02/2019 02:18, Alex Williamson wrote: > >>>> On Thu, 7 Feb 2019 15:43:18 +1100 > >>>> Alexey Kardashevskiy <a...@ozlabs.ru> 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. > > This is accounting for TCE table to cover GPU RAM, not for GPU RAM itself.
Ah, ok, that makes sense then
> So I am asking libvirt to add 128GB/64k*8=16MB to the locked_vm. It
> already does so for the guest RAM.
That seems reasonable. IIRC we already have some slop in the amount
of locked vm that libvirt allocates; not sure if it'll be enough as
is.
--
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
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