[PATCH v2 1/1] monitor: Support specified vCPU registers

[zhenwei pi <pizhenwei@bytedance.com>]

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>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
---
 hmp-commands-info.hx |  7 ++++---
 monitor/misc.c       | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/hmp-commands-info.hx b/hmp-commands-info.hx
index 3ffa24bd67..7a00b4ded3 100644
--- a/hmp-commands-info.hx
+++ b/hmp-commands-info.hx
@@ -100,9 +100,10 @@ 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: all - show register info for all cpus;"
+                      " vcpu: specific vCPU to query)",
         .cmd        = hmp_info_registers,
     },
 
diff --git a/monitor/misc.c b/monitor/misc.c
index 3d2312ba8d..8e1d4840f2 100644
--- a/monitor/misc.c
+++ b/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) {
@@ -314,6 +315,23 @@ static void hmp_info_registers(Monitor *mon, const QDict *qdict)
             monitor_printf(mon, "\nCPU#%d\n", cs->cpu_index);
             cpu_dump_state(cs, NULL, CPU_DUMP_FPU);
         }
+    } else if (vcpu >= 0) {
+        CPUState *target_cs = NULL;
+
+        CPU_FOREACH(cs) {
+            if (cs->cpu_index == vcpu) {
+                target_cs = cs;
+                break;
+            }
+        }
+
+        if (!target_cs) {
+            monitor_printf(mon, "CPU#%d not available\n", vcpu);
+            return;
+        }
+
+        monitor_printf(mon, "\nCPU#%d\n", target_cs->cpu_index);
+        cpu_dump_state(target_cs, NULL, CPU_DUMP_FPU);
     } else {
         cs = mon_get_cpu(mon);
 
-- 
2.20.1