add update indicator to vcpu_runstate_info

add update indicator to vcpu_runstate_info

There has been a report about incorrect vruntime accounting in Linux
guests under Xen. A Linux kernel with CONFIG_PARAVIRT_TIME_ACCOUNTING
set is capable to do correct vruntime accounting, but this would
require the kernel to be able to read the runstate data of other cpus.

A guest mapping vcpu_runstate_info into its memory can't read this
information from another cpu but the one the data is referring to.
Reason is there is no reliable way for the guest to detect a concurrent
data update by the hypervisor. In order to support reading another
vcpu's mapped vcpu_runstate_info an indicator for an occurring update
of the runstate information is needed.

Add the possibility to activate setting this indicator in the highest
bit of state_entry_time via a vm_assist hypercall. When activated the
update indicator will be set before the runstate information is
modified in guest memory and it will be reset after modification is
done. As state_entry_time is guaranteed to be different after each
update the guest can detect any update (either in progress or while
reading the runstate data) by comparing state_entry_time before and
after reading runstate data: in case the values differ or the update
indicator was set the data might be inconsistent and should be reread.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>