What’s the internal details for ‘-startdate’ option in ‘qemu-kvm’
When you run KVM-based virtual machine, you can specify ‘-startdate’ option. What format we can use and what actions are happening inside the qemu-kvm.
Options are checked in main() function in vl.c.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | case QEMU_OPTION_startdate: { struct tm tm; time_t rtc_start_date; if (!strcmp(optarg, "now")) { rtc_date_offset = -1; } else { if (sscanf(optarg, "%d-%d-%dT%d:%d:%d", &tm.tm_year, &tm.tm_mon, &tm.tm_mday, &tm.tm_hour, &tm.tm_min, &tm.tm_sec) == 6) { /* OK */ } else if (sscanf(optarg, "%d-%d-%d", &tm.tm_year, &tm.tm_mon, &tm.tm_mday) == 3) { tm.tm_hour = 0; tm.tm_min = 0; tm.tm_sec = 0; } else { goto date_fail; } tm.tm_year -= 1900; tm.tm_mon--; rtc_start_date = mktimegm(&tm); if (rtc_start_date == -1) { date_fail: fprintf(stderr, "Invalid date format. Valid format are:\n" "'now' or '2006-06-17T16:01:21' or '2006-06-17'\n"); exit(1); } rtc_date_offset = time(NULL) - rtc_start_date; } } break; |
rtc_date_offset is declared as a global variable in vl.c.
1 | static int rtc_date_offset = -1; /* -1 means no change */ |
This variable is used to use time information in virtual machine as shown below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 | /***********************************************************/ /* host time/date access */ void qemu_get_timedate(struct tm *tm, int offset) { time_t ti; struct tm *ret; time(&ti); ti += offset; if (rtc_date_offset == -1) { if (rtc_utc) ret = gmtime(&ti); else ret = localtime(&ti); } else { ti -= rtc_date_offset; ret = gmtime(&ti); } memcpy(tm, ret, sizeof(struct tm)); } int qemu_timedate_diff(struct tm *tm) { time_t seconds; if (rtc_date_offset == -1) if (rtc_utc) seconds = mktimegm(tm); else seconds = mktime(tm); else seconds = mktimegm(tm) + rtc_date_offset; return seconds - time(NULL); } |
In here, we can see that the time information is basically come from system OS’s time facilities such as gmtime and localtime. This information can be modified if customer wants to.
