[Qemu-devel] [PATCH RFC 02/16] vl: smp: add checks for maxcpus based topologies

[Andrew Jones]

smp_parse computes missing smp options. Unfortunately cores and
threads are computed by dividing smp_cpus, instead of max_cpus.
This is incorrect because the topology doesn't leave room for
hotplug. More unfortunately, we can't change it easily, as doing
so would impact existing command lines. This patch adds a warning
when the topology doesn't add up, and then checks that the topology
at least computes when sockets are recalculated. If not, then it
does fail.

Adding the new failure is justified by the fact that we don't
store the number of input sockets, and thus all consumers of
cpu topology information recalculate it. If they choose to
(correctly) calculate it based on maxcpus, then we need to
guard them against building topologies which provide more cpu
slots than are the maximum allowed cpus.

Signed-off-by: Andrew Jones <address@hidden>
---
 vl.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/vl.c b/vl.c
index 7b96e787922f9..8d482cb1bf020 100644
--- a/vl.c
+++ b/vl.c
@@ -1227,6 +1227,7 @@ static void smp_parse(QemuOpts *opts)
         unsigned sockets = qemu_opt_get_number(opts, "sockets", 0);
         unsigned cores   = qemu_opt_get_number(opts, "cores", 0);
         unsigned threads = qemu_opt_get_number(opts, "threads", 0);
+        bool sockets_input = sockets > 0;
 
         /* compute missing values, prefer sockets over cores over threads */
         if (cpus == 0 || sockets == 0) {
@@ -1269,6 +1270,24 @@ static void smp_parse(QemuOpts *opts)
             exit(1);
         }
 
+        if (sockets_input && sockets * cores * threads != max_cpus) {
+            unsigned sockets_rounded = DIV_ROUND_UP(max_cpus, cores * threads);
+
+            error_report("warning: cpu topology: "
+                         "sockets (%u) * cores (%u) * threads (%u) != "
+                         "maxcpus (%u). Trying sockets=%u.",
+                         sockets, cores, threads, max_cpus, sockets_rounded);
+            sockets = sockets_rounded;
+
+            if (sockets * cores * threads > max_cpus) {
+                error_report("cpu topology: "
+                             "sockets (%u) * cores (%u) * threads (%u) > "
+                             "maxcpus (%u)",
+                             sockets, cores, threads, max_cpus);
+                exit(1);
+            }
+        }
+
         smp_cpus = cpus;
         smp_cores = cores;
         smp_threads = threads;
-- 
2.4.11