[libvirt] [RFC PATCH 1/2] nodeinfo: Reflect guest usable host topology on PPC64
Today, the topology displayed by nodeinfo/capabilities is reflecting the host
topology irrespective the subcore_per_core setting.
This is not of much use as user cannot determine what is the maximum
possible threads_per_core value he can use for a guest topology.
For subcore_per_core = 1, the guest topology can have threads_per_core=1,2,4
or 8. Same way for subcore_per_core = 2, the guest toplogy can have
threads_per_core=1,2 or 4; and for subcore_per_core = 4, guest threads_per_core
can be 1 or 2 only.
Incidentally, the topology displayed in subcore_per_core=1 is correct and is
usable today. If we can display the guest usable topology info for
subcore_per_core=2 and 4 too, then it can help making guest topology decisions.
Signed-off-by: Shivaprasad G Bhat <sbhat(a)linux.vnet.ibm.com>
---
src/nodeinfo.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/src/nodeinfo.c b/src/nodeinfo.c
index 77ea155..531e0ee 100644
--- a/src/nodeinfo.c
+++ b/src/nodeinfo.c
@@ -849,6 +849,23 @@ linuxNodeInfoCPUPopulate(const char *sysfs_prefix,
goto cleanup;
}
+ /* On PPC if host is in a valid sub core mode, the maximum supported
+ * threads_per_core for a guest is decided by the maximum
+ * threads_per_subcore.
+ * In other words on P8, if the subcores-per-core=1,
+ * the guest can have cpu topology with threads_per_core=1, 2, 4 or 8
+ * and for subcores-per-core=2, the guest can have
+ * threads_per_core=1, 2 or 4 only. Same way, for subcores-per-core=4
+ * guest can have threads_per_core=1 or 2 only.
+ * On a valid kvm host reflecddt the guest supportable topology instead
+ * of host topology.
+ */
+ if (threads_per_subcore) {
+ int subcores_per_core = nodeinfo->threads/threads_per_subcore;
+ nodeinfo->cores *= subcores_per_core;
+ nodeinfo->threads /= subcores_per_core;
+ }
+
/* Now check if the topology makes sense. There are machines that don't
* expose their real number of nodes or for example the AMD Bulldozer
* architecture that exposes their Clustered integer core modules as both