monitor: Support specified vCPU registers

Originally we have to get all the vCPU registers and parse the
specified one. To improve the performance of this usage, allow user
specified vCPU id to query registers.

Run a VM with 16 vCPU, use bcc tool to track the latency of
'hmp_info_registers':
'info registers -a' uses about 3ms;
'info registers 12' uses about 150us.

Cc: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20220802073720.1236988-2-pizhenwei@bytedance.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

hmp-commands-info.hx

@@ -100,9 +100,11 @@ ERST
 
     {
         .name       = "registers",
-        .args_type  = "cpustate_all:-a",
-        .params     = "[-a]",
-        .help       = "show the cpu registers (-a: all - show register info for all cpus)",
+        .args_type  = "cpustate_all:-a,vcpu:i?",
+        .params     = "[-a|vcpu]",
+        .help       = "show the cpu registers (-a: show register info for all cpus;"
+                      " vcpu: specific vCPU to query; show the current CPU's registers if"
+                      " no argument is specified)",
         .cmd        = hmp_info_registers,
     },
 

monitor/misc.c

@@ -307,6 +307,7 @@ int monitor_get_cpu_index(Monitor *mon)
 static void hmp_info_registers(Monitor *mon, const QDict *qdict)
 {
     bool all_cpus = qdict_get_try_bool(qdict, "cpustate_all", false);
+    int vcpu = qdict_get_try_int(qdict, "vcpu", -1);
     CPUState *cs;
 
     if (all_cpus) {
@@ -315,13 +316,18 @@ static void hmp_info_registers(Monitor *mon, const QDict *qdict)
             cpu_dump_state(cs, NULL, CPU_DUMP_FPU);
         }
     } else {
-        cs = mon_get_cpu(mon);
+        cs = vcpu >= 0 ? qemu_get_cpu(vcpu) : mon_get_cpu(mon);
 
         if (!cs) {
-            monitor_printf(mon, "No CPU available\n");
+            if (vcpu >= 0) {
+                monitor_printf(mon, "CPU#%d not available\n", vcpu);
+            } else {
+                monitor_printf(mon, "No CPU available\n");
+            }
             return;
         }
 
+        monitor_printf(mon, "\nCPU#%d\n", cs->cpu_index);
         cpu_dump_state(cs, NULL, CPU_DUMP_FPU);
     }
 }