[RFC,V4,2/6] icount: Add align option to icount

diff --git a/cpus.c b/cpus.c
index dcca96a..34cc4c8 100644
--- a/cpus.c
+++ b/cpus.c
@@ -443,25 +443,30 @@  static const VMStateDescription vmstate_timers = {
 void configure_icount(QemuOpts *opts, Error **errp)
 {
     const char *option;
+    char *rem_str = NULL;
 
     seqlock_init(&timers_state.vm_clock_seqlock, NULL);
     vmstate_register(NULL, 0, &vmstate_timers, &timers_state);
     option = qemu_opt_get(opts, "shift");
     if (!option) {
+        if (qemu_opt_get(opts, "align") != NULL) {
+            error_setg(errp, "Please specify shift option when using align");
+        }
         return;
     }
-    /* When using -icount shift, the shift option will be
-       misinterpreted as a boolean */
-    if (strcmp(option, "on") == 0 || strcmp(option, "off") == 0) {
-        error_setg(errp, "The shift option must be a number or auto");
-    }
-
+    icount_align_option = qemu_opt_get_bool(opts, "align", false);
     icount_warp_timer = timer_new_ns(QEMU_CLOCK_REALTIME,
                                           icount_warp_rt, NULL);
     if (strcmp(option, "auto") != 0) {
-        icount_time_shift = strtol(option, NULL, 0);
+        errno = 0;
+        icount_time_shift = strtol(option, &rem_str, 0);
+        if (errno != 0 || *rem_str != '\0' || !strlen(option)) {
+            error_setg(errp, "icount: Invalid shift value");
+        }
         use_icount = 1;
         return;
+    } else if (icount_align_option) {
+        error_setg(errp, "shift=auto and align=on are incompatible");
     }
 
     use_icount = 2;
diff --git a/include/qemu-common.h b/include/qemu-common.h
index cc346ec..860bb15 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -108,6 +108,7 @@  static inline char *realpath(const char *path, char *resolved_path)
 /* icount */
 void configure_icount(QemuOpts *opts, Error **errp);
 extern int use_icount;
+extern int icount_align_option;
 
 #include "qemu/osdep.h"
 #include "qemu/bswap.h"
diff --git a/qemu-options.hx b/qemu-options.hx
index 143def4..379d932 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3011,9 +3011,9 @@  re-inject them.
 ETEXI
 
 DEF("icount", HAS_ARG, QEMU_OPTION_icount, \
-    "-icount [shift=N|auto]\n" \
+    "-icount [shift=N|auto][,align=on|off]\n" \
     "                enable virtual instruction counter with 2^N clock ticks per\n" \
-    "                instruction\n", QEMU_ARCH_ALL)
+    "                instruction and enable aligning the host and virtual clocks\n", QEMU_ARCH_ALL)
 STEXI
 @item -icount [shift=@var{N}|auto]
 @findex -icount
@@ -3026,6 +3026,17 @@  Note that while this option can give deterministic behavior, it does not
 provide cycle accurate emulation.  Modern CPUs contain superscalar out of
 order cores with complex cache hierarchies.  The number of instructions
 executed often has little or no correlation with actual performance.
+
+@option{align=on} will activate the delay algorithm which will try to
+to synchronise the host clock and the virtual clock. The goal is to
+have a guest running at the real frequency imposed by the shift option.
+Whenever the guest clock is behind the host clock and if
+@option{align=on} is specified then we print a messsage to the user
+to inform about the delay.
+Currently this option does not work when @option{shift} is @code{auto}.
+Note: The sync algorithm will work for those shift values for which
+the guest clock runs ahead of the host clock. Typically this happens
+when the shift value is high (how high depends on the host machine).
 ETEXI
 
 DEF("watchdog", HAS_ARG, QEMU_OPTION_watchdog, \
diff --git a/vl.c b/vl.c
index 103027f..d270070 100644
--- a/vl.c
+++ b/vl.c
@@ -183,6 +183,7 @@  uint8_t *boot_splash_filedata;
 size_t boot_splash_filedata_size;
 uint8_t qemu_extra_params_fw[2];
 
+int icount_align_option;
 typedef struct FWBootEntry FWBootEntry;
 
 struct FWBootEntry {
@@ -546,6 +547,9 @@  static QemuOptsList qemu_icount_opts = {
         {
             .name = "shift",
             .type = QEMU_OPT_STRING,
+        }, {
+            .name = "align",
+            .type = QEMU_OPT_BOOL,
         },
         { /* end of list */ }
     },