vmware: add GETTIME backdoor command (23)#1590
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Returns the host's wall-clock time: EAX = seconds since the Unix epoch (UTC), EBX = remaining microseconds, ECX = maximum time lag in microseconds, EDX = host offset from UTC in minutes (east positive). Guests only read the RTC at boot, so a VM resumed from a saved state keeps running with the clock it was saved with. With this command a small guest agent can periodically compare its clock against the host and correct it without a reboot.
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Just like the other VMWare-related PRs, no hard feelings if you don't want them - I just want to upstream stuff I've done for Windows. This one implements implements VMware backdoor command 23 (
GETTIME) on port 0x5658.Return values in registers after the
IN EAX, DX:Guests read the RTC only at boot. A VM resumed from a saved state continues running with whatever clock it was saved with, and the guest has no way to learn the correct time without rebooting.
With GETTIME, a small guest agent can issue a single backdoor call on a timer and compare its own
time()against the host — no kernel driver, no full VMware Tools stack. The protocol is simple enough for a hand-rolled 16/32-bit DOS or Win9x TSR.Notes on GETTIMEFULL
VMware deprecated GETTIME in favour of command 35 (
GETTIMEFULL) because EAX wraps as a signed value in 2038. Read as unsigned (which is what>>> 0gives JS, and what 32-bit C gives withunsigned long), EAX is valid until 2106. GETTIME is simpler for the guest to consume and is still the first thing open-vm-tools'Backdoor_GetTimewrapper tries. I think for our purposes her eis is probably fine :)