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The PL031 RTC tracks the difference between the guest RTC
and the host RTC using a tick_offset field. For migration,
however, we currently always migrate the offset between
the guest and the vm_clock, even if the RTC clock is not
the same as the vm_clock; this was an attempt to retain
migration backwards compatibility.
Unfortunately this results in the RTC behaving oddly across
a VM state save and restore -- since the VM clock stands still
across save-then-restore, regardless of how much real world
time has elapsed, the guest RTC ends up out of sync with the
host RTC in the restored VM.
Fix this by migrating the raw tick_offset. To retain migration
compatibility as far as possible, we have a new property
migrate-tick-offset; by default this is 'true' and we will
migrate the true tick offset in a new subsection; if the
incoming data has no subsection we fall back to the old
vm_clock-based offset information, so old->new migration
compatibility is preserved. For complete new->old migration
compatibility, the property is set to 'false' for 4.0 and
earlier machine types (this will only affect 'virt-4.0'
and below, as none of the other pl031-using machines are
versioned).
Reported-by: Russell King <rmk@armlinux.org.uk>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 20190709143912.28905-1-peter.maydell@linaro.org
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