target/arm: Saturate L2CTLR_EL1 core count field rather than overflowing

The IMPDEF sysreg L2CTLR_EL1 found on the Cortex-A35, A53, A57, A72
and which we (arguably dubiously) also provide in '-cpu max' has a
2 bit field for the number of processors in the cluster. On real
hardware this must be sufficient because it can only be configured
with up to 4 CPUs in the cluster. However on QEMU if the board code
does not explicitly configure the code into clusters with the right
CPU count we default to "give the value assuming that all CPUs in
the system are in a single cluster", which might be too big to fit
in the field.

Instead of just overflowing this 2-bit field, saturate to 3 (meaning
"4 CPUs", so at least we don't overwrite other fields in the register.
It's unlikely that any guest code really cares about the value in
this field; at least, if it does it probably also wants the system
to be more closely matching real hardware, i.e. not to have more
than 4 CPUs.

This issue has been present since the L2CTLR was first added in
commit 377a44ec8f back in 2014. It was only noticed because
Coverity complains (CID 1509227) that the shift might overflow 32 bits
and inadvertently sign extend into the top half of the 64 bit value.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230512170223.3801643-2-peter.maydell@linaro.org

target/arm/cortex-regs.c

@@ -15,8 +15,15 @@ static uint64_t l2ctlr_read(CPUARMState *env, const ARMCPRegInfo *ri)
 {
     ARMCPU *cpu = env_archcpu(env);
 
-    /* Number of cores is in [25:24]; otherwise we RAZ */
-    return (cpu->core_count - 1) << 24;
+    /*
+     * Number of cores is in [25:24]; otherwise we RAZ.
+     * If the board didn't configure the CPUs into clusters,
+     * we default to "all CPUs in one cluster", which might be
+     * more than the 4 that the hardware permits and which is
+     * all you can report in this two-bit field. Saturate to
+     * 0b11 (== 4 CPUs) rather than overflowing the field.
+     */
+    return MIN(cpu->core_count - 1, 3) << 24;
 }
 
 static const ARMCPRegInfo cortex_a72_a57_a53_cp_reginfo[] = {