virnetdevbandwidth.c: Put a limit to "quantum"

The "quantum" attribute of HTB is documented as:

  Number of bytes to serve from this class before the scheduler
  moves to the next class.

Since v1.3.2-rc1~225 we compute what we think is the appropriate
value and pass it on the TC command line. But kernel and
subsequently TC use uint32_t to store this value. If we compute
value outside of this type then TC fails and prints usage which
we then interpret as an error message. Needlessly long error
message. While there's not much we can do about the latter, we
can put a cap on the value and stop tickling this behavior of TC.

Fixes: 065054daa7
Resolves: https://issues.redhat.com/browse/RHEL-34112
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/util/virnetdevbandwidth.c

@@ -46,6 +46,7 @@ virNetDevBandwidthCmdAddOptimalQuantum(virCommand *cmd,
                                        const virNetDevBandwidthRate *rate)
 {
     const unsigned long long mtu = 1500;
+    const unsigned long long r2q_limit = UINT32_MAX;
     unsigned long long r2q;
 
     /* When two or more classes compete for unused bandwidth they are each
@@ -60,6 +61,11 @@ virNetDevBandwidthCmdAddOptimalQuantum(virCommand *cmd,
     if (!r2q)
         r2q = 1;
 
+    /* But there's an internal limit in TC (well, kernel's implementation of
+     * HTB) for quantum: it has to fit into u32. Put a cap there. */
+    if (r2q > r2q_limit)
+        r2q = r2q_limit;
+
     virCommandAddArg(cmd, "quantum");
     virCommandAddArgFormat(cmd, "%llu", r2q);
 }