On Fri, Oct 08, 2021 at 10:51:24AM +1100, Gavin Shan wrote:
On 10/6/21 10:56 PM, Andrew Jones wrote:
On Wed, Oct 06, 2021 at 10:03:25PM +1100, Gavin Shan wrote:
On 10/6/21 9:35 PM, Andrew Jones wrote:
On Wed, Oct 06, 2021 at 06:22:08PM +0800, Gavin Shan wrote:
The following option is used to specify the distance map. It's
possible the option isn't provided by user. In this case, the
distance map isn't populated and exposed to platform. On the
other hand, the empty NUMA node, where no memory resides, is
allowed on ARM64 virt platform. For these empty NUMA nodes,
their corresponding device-tree nodes aren't populated, but
their NUMA IDs should be included in the "/distance-map"
device-tree node, so that kernel can probe them properly if
device-tree is used.
-numa,dist,src=<numa_id>,dst=<numa_id>,val=<distance>
So when user doesn't specify distance map, we need to generate
the default distance map, where the local and remote distances
are 10 and 20 separately. This adds an extra parameter to the
exiting complete_init_numa_distance() to generate the default
distance map for this case.
Signed-off-by: Gavin Shan <gshan@redhat.com>
---
hw/core/numa.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/hw/core/numa.c b/hw/core/numa.c
index 510d096a88..fdb3a4aeca 100644
--- a/hw/core/numa.c
+++ b/hw/core/numa.c
@@ -594,7 +594,7 @@ static void validate_numa_distance(MachineState *ms)
}
}
-static void complete_init_numa_distance(MachineState *ms)
+static void complete_init_numa_distance(MachineState *ms, bool is_default)
{
int src, dst;
NodeInfo *numa_info = ms->numa_state->nodes;
@@ -609,6 +609,8 @@ static void complete_init_numa_distance(MachineState *ms)
if (numa_info[src].distance[dst] == 0) {
if (src == dst) {
numa_info[src].distance[dst] = NUMA_DISTANCE_MIN;
+ } else if (is_default) {
+ numa_info[src].distance[dst] = NUMA_DISTANCE_DEFAULT;
} else {
numa_info[src].distance[dst] =
numa_info[dst].distance[src];
}
@@ -716,13 +718,20 @@ void numa_complete_configuration(MachineState *ms)
* A->B != distance B->A, then that means the distance table is
* asymmetric. In this case, the distances for both directions
* of all node pairs are required.
+ *
+ * The default node pair distances, which are 10 and 20 for the
+ * local and remote nodes separatly, are provided if user doesn't
+ * specify any node pair distances.
*/
if (ms->numa_state->have_numa_distance) {
/* Validate enough NUMA distance information was provided. */
validate_numa_distance(ms);
/* Validation succeeded, now fill in any missing distances. */
- complete_init_numa_distance(ms);
+ complete_init_numa_distance(ms, false);
+ } else {
+ complete_init_numa_distance(ms, true);
+ ms->numa_state->have_numa_distance = true;
}
}
}
--
2.23.0
With this patch we'll always generate a distance map when there's a numa
config now. Is there any reason a user would not want to do that? I.e.
should we still give the user the choice of presenting a distance map?
Also, does the addition of a distance map in DTs for compat machine types
matter?
Otherwise patch looks good to me.
Users needn't specify the distance map when the default one in kernel,
whose distances are 10 and 20 for local and remote nodes in linux for
all architectures and machines, is used. The following option is still
usable to specify the distance map.
-numa,dist,src=<numa_id>,dst=<numa_id>,val=<distance>
When the empty NUMA nodes are concerned, the distance map is mandatory
because their NUMA IDs are identified from there. So we always generate
the distance map as this patch does :)
Yup, I knew all that already :-) I'm asking if we want to ensure the user
can still control whether or not this distance map is generated at all. If
a user doesn't want empty numa nodes or a distance map, then, with this
patch, they cannot avoid the map's generation. That configurability
question also relates to machine compatibility. Do we want to start
generating this distance map on old, numa configured machine types? This
patch will do that too.
But, it might be OK to just start generating this new DT node for all numa
configured machine types and not allow the user to opt out. I do know that
we allow hardware descriptions to be changed without compat code. Also, a
disable-auto-distance-map option may be considered useless and therefore
not worth maintaining. The conservative in me says it's worth debating
these things first though.
(Note, empty numa nodes have never worked with QEMU, so it's OK to start
erroring out when empty numa nodes and a disable-auto-distance-map option
are given together.)
Sorry for the delay. I didn't fully understand "machine compatibility" even
after checking the code around. Could you please provide more details? I'm
not sure if the enforced distance-map for empty NUMA nodes will cause any
issues?
On QEMU, currently booting/running VMs on machine type X should not notice
when QEMU has been updated and they are still boot with machine type X.
That's what the "compat machine types" stuff means and what I'm referring
to above. I think it may be fine to boot a VM that never had a
distance-map before on an updated QEMU with machine type X and suddenly
get a distance-map, because we claim this is similar to a firmware update
that will change hardware descriptions on reboot. We expect guest kernels
to be tolerant of that. That said, there's always some risk, so we need
to consciously make that decision. Also, if we choose to expose a switch
to disable to the auto-distance-map to the user, then it's pretty trivial
to automatically set that on older machine types in order to avoid the
concern. So, do we think we need to expose a disable-auto-distance-map
type of option? Or would that be a useless burden? Also, if the decision
is to not worry about it, then the commit message should be updated to
add the rationale for that decision.
Yes, the empty NUMA node never worked with QEMU if device-tree is used.
We still need to figure out a way to support memory hotplug through
device-tree, similar thing as to what IBM's pSeries platform has.
That's for the guest kernel to figure out. I doubt it'll be a high
priority, though, because, as you've shown below, memory hotplug works
with ACPI, which is what Arm servers use. I don't expect smaller DT
platforms to care much about memory hotplug.