x86/time: fix gtime_to_gtsc for vtsc=1 PV guests

x86/time: fix gtime_to_gtsc for vtsc=1 PV guests

For vtsc=1 PV guests, rdtsc is trapped and calculated from get_s_time()
using gtime_to_gtsc. Similarly the tsc_timestamp, part of struct
vcpu_time_info, is calculated from stime_local_stamp using
gtime_to_gtsc.

However gtime_to_gtsc can return 0, if time < vtsc_offset, which can
actually happen when gtime_to_gtsc is called passing stime_local_stamp
(the caller function is __update_vcpu_system_time).

In that case the pvclock protocol doesn't work properly and the guest is
unable to calculate the system time correctly. As a consequence when the
guest tries to set a timer event (for example calling the
VCPUOP_set_singleshot_timer hypercall), the event will be in the past
causing Linux to hang.

The purpose of the pvclock protocol is to allow the guest to calculate
the system_time in nanosec correctly. The guest calculates as follow:

  from_vtsc_scale(rdtsc - vcpu_time_info.tsc_timestamp) + vcpu_time_info.system_time

Given that with vtsc=1:
  rdtsc = to_vtsc_scale(NOW() - vtsc_offset)
  vcpu_time_info.tsc_timestamp = to_vtsc_scale(vcpu_time_info.system_time - vtsc_offset)

The expression evaluates to NOW(), which is what we want.  However when
stime_local_stamp < vtsc_offset, vcpu_time_info.tsc_timestamp is
actually 0. As a consequence the calculated overall system_time is not
correct.

This patch fixes the issue by letting gtime_to_gtsc return a negative
integer in the form of a wrapped around unsigned integer, thus when the
guest subtracts vcpu_time_info.tsc_timestamp from rdtsc will calculate
the right value.

Signed-off-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>