[Qemu-devel] Partial NUMA config

[Alexey Kardashevskiy]

Hi Igor,

ec78f8114bc4c1 "numa: use possible_cpus for not mapped CPUs check" added a
warning about "All CPU(s) up to maxcpus should be described in NUMA config,
ability to start up with partial NUMA mappings is obsoleted and will be
removed in future" and this is printed when I add a numa node without
attached CPU like this:

-numa node,nodeid=0,cpus=0,mem=4G
-numa node,nodeid=1,mem=131072M

And the reason for this command line is that I am trying to pass some dodgy
host RAM (actually belongs to a GPU but directly accessible via a fast
NVLink, not PCI fabric) which let's say is equally far from all CPUs, at
least in the host's NUMA config this memory is also not bound to any CPU:

address@hidden ~]$ ssh yc02goos numactl -H
available: 8 nodes (0,8,250-255)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
node 0 size: 261735 MB
node 0 free: 258932 MB
node 8 cpus: 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106
107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125
126 127
node 8 size: 261739 MB
node 8 free: 261414 MB
node 250 cpus:
node 250 size: 15360 MB
node 250 free: 15359 MB
node 251 cpus:
node 251 size: 0 MB
node 251 free: 0 MB
node 252 cpus:
node 252 size: 15360 MB
node 252 free: 15359 MB
node 253 cpus:
node 253 size: 15360 MB
node 253 free: 15359 MB
node 254 cpus:
node 254 size: 15360 MB
node 254 free: 15359 MB
node 255 cpus:
node 255 size: 15360 MB
node 255 free: 15359 MB
node distances:
node   0   8  250  251  252  253  254  255
  0:  10  40  80  80  80  80  80  80
  8:  40  10  80  80  80  80  80  80
 250:  80  80  10  80  80  80  80  80
 251:  80  80  80  10  80  80  80  80
 252:  80  80  80  80  10  80  80  80
 253:  80  80  80  80  80  10  80  80
 254:  80  80  80  80  80  80  10  80
 255:  80  80  80  80  80  80  80  10


I am not sure I'll progress far enough to get this working with VFIO but if
I do, I'd like to keep an ability to have such a partial config in the
future. What was the reason for this warning in the first place?


-- 
Alexey