target/arm: Don't get MDCR_EL2 in pmu_counter_enabled() before checki…

…ng ARM_FEATURE_PMU

It doesn't make sense to read the value of MDCR_EL2 on a non-A-profile
CPU, and in fact if you try to do it we will assert:

eurecom-s3#6  0x00007ffff4b95e96 in __GI___assert_fail
    (assertion=0x5555565a8c70 "!arm_feature(env, ARM_FEATURE_M)", file=0x5555565a6e5c "../../target/arm/helper.c", line=12600, function=0x5555565a9560 <__PRETTY_FUNCTION__.0> "arm_security_space_below_el3") at ./assert/assert.c:101
eurecom-s3#7  0x0000555555ebf412 in arm_security_space_below_el3 (env=0x555557bc8190) at ../../target/arm/helper.c:12600
eurecom-s3#8  0x0000555555ea6f89 in arm_is_el2_enabled (env=0x555557bc8190) at ../../target/arm/cpu.h:2595
eurecom-s3#9  0x0000555555ea942f in arm_mdcr_el2_eff (env=0x555557bc8190) at ../../target/arm/internals.h:1512

We might call pmu_counter_enabled() on an M-profile CPU (for example
from the migration pre/post hooks in machine.c); this should always
return false because these CPUs don't set ARM_FEATURE_PMU.

Avoid the assertion by not calling arm_mdcr_el2_eff() before we
have done the early return for "PMU not present".

This fixes an assertion failure if you try to do a loadvm or
savevm for an M-profile board.

Cc: [email protected]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2155
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: [email protected]
(cherry picked from commit ac1d88e)
Signed-off-by: Michael Tokarev <[email protected]>

target/arm/helper.c

@@ -1169,13 +1169,21 @@ static bool pmu_counter_enabled(CPUARMState *env, uint8_t counter)
     bool enabled, prohibited = false, filtered;
     bool secure = arm_is_secure(env);
     int el = arm_current_el(env);
-    uint64_t mdcr_el2 = arm_mdcr_el2_eff(env);
-    uint8_t hpmn = mdcr_el2 & MDCR_HPMN;
+    uint64_t mdcr_el2;
+    uint8_t hpmn;
 
+    /*
+     * We might be called for M-profile cores where MDCR_EL2 doesn't
+     * exist and arm_mdcr_el2_eff() will assert, so this early-exit check
+     * must be before we read that value.
+     */
     if (!arm_feature(env, ARM_FEATURE_PMU)) {
         return false;
     }
 
+    mdcr_el2 = arm_mdcr_el2_eff(env);
+    hpmn = mdcr_el2 & MDCR_HPMN;
+
     if (!arm_feature(env, ARM_FEATURE_EL2) ||
             (counter < hpmn || counter == 31)) {
         e = env->cp15.c9_pmcr & PMCRE;