arm: quick hack to allow a greater number of CPUs to a guest OS

This is a quick hack to communicate a greater number of CPUs to a guest OS via
the ARM A9 SCU config register. Some OSes (Linux) just look at the bottom field
to count CPUs and with a small change can look at bits [3:0] to learn about up
to 16 CPUs.

Very much unsupported (and contains warning messages as such) but useful for
running 8 core sims without hardwiring CPU count in the guest OS.

1 files changed, 10 insertions, 1 deletions

diff --git a/src/dev/arm/a9scu.cc b/src/dev/arm/a9scu.cc
index efb27daf9..dc012a036 100644
--- a/src/dev/arm/a9scu.cc
+++ b/src/dev/arm/a9scu.cc
@@ -62,7 +62,16 @@ A9SCU::read(PacketPtr pkt)
         pkt->set(1); // SCU already enabled
         break;
       case Config:
-        assert(sys->numContexts() <= 4);
+        /* Without making a completely new SCU, we can use the core count field
+         * as 4 bits and inform the OS of up to 16 CPUs.  Although the core
+         * count is technically bits [1:0] only, bits [3:2] are SBZ for future
+         * expansion like this.
+         */
+        if (sys->numContexts() > 4) {
+            warn_once("A9SCU with >4 CPUs is unsupported\n");
+            if (sys->numContexts() > 15)
+                fatal("Too many CPUs (%d) for A9SCU!\n", sys->numContexts());
+        }
         int smp_bits, core_cnt;
         smp_bits = power(2,sys->numContexts()) - 1;
         core_cnt = sys->numContexts() - 1;