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vl: improve/fix documentation related to RTC function 

Documentation describing -rtc option updated to better match current
implementation and highlight some important specifics.

Signed-off-by: Artem Pisarenko <artem.k.pisarenko@gmail.com>
Message-Id: <1b245c6c0803d4bf11dcbf9eb32f34af8c2bd0b4.1539846575.git.artem.k.pisarenko@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Artem Pisarenko 2018-10-18 13:12:52 +06:00 committed by Paolo Bonzini
parent 92cc3aaa1f
commit 238d1240d9
1 changed files with 9 additions and 5 deletions
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14
qemu-options.hx
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@ -3458,25 +3458,29 @@ HXCOMM Silently ignored for compatibility
DEF("clock", HAS_ARG, QEMU_OPTION_clock, "", QEMU_ARCH_ALL)
DEF("rtc", HAS_ARG, QEMU_OPTION_rtc, \
"-rtc [base=utc|localtime|date][,clock=host|rt|vm][,driftfix=none|slew]\n" \
"-rtc [base=utc|localtime|<datetime>][,clock=host|rt|vm][,driftfix=none|slew]\n" \
" set the RTC base and clock, enable drift fix for clock ticks (x86 only)\n",
QEMU_ARCH_ALL)
STEXI
@item -rtc [base=utc|localtime|@var{date}][,clock=host|vm][,driftfix=none|slew]
@item -rtc [base=utc|localtime|@var{datetime}][,clock=host|rt|vm][,driftfix=none|slew]
@findex -rtc
Specify @option{base} as @code{utc} or @code{localtime} to let the RTC start at the current
UTC or local time, respectively. @code{localtime} is required for correct date in
MS-DOS or Windows. To start at a specific point in time, provide @var{date} in the
MS-DOS or Windows. To start at a specific point in time, provide @var{datetime} in the
format @code{2006-06-17T16:01:21} or @code{2006-06-17}. The default base is UTC.
By default the RTC is driven by the host system time. This allows using of the
RTC as accurate reference clock inside the guest, specifically if the host
time is smoothly following an accurate external reference clock, e.g. via NTP.
If you want to isolate the guest time from the host, you can set @option{clock}
to @code{rt} instead. To even prevent it from progressing during suspension,
you can set it to @code{vm}.
to @code{rt} instead, which provides a host monotonic clock if host support it.
To even prevent the RTC from progressing during suspension, you can set @option{clock}
to @code{vm} (virtual clock). @samp{clock=vm} is recommended especially in
icount mode in order to preserve determinism; however, note that in icount mode
the speed of the virtual clock is variable and can in general differ from the
host clock.
Enable @option{driftfix} (i386 targets only) if you experience time drift problems,
specifically with Windows' ACPI HAL. This option will try to figure out how