Re: [Qemu-devel] [PATCH qemu 0/3] spapr_pci, vfio: NVIDIA V100 + P9 passthrough

[Alex Williamson]

On Thu, 7 Feb 2019 15:43:18 +1100
Alexey Kardashevskiy <address@hidden> 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,

Alex