[Xen-changelog] [xen stable-4.6] x86/time: fix gtime_to_gtsc for vtsc=1 PV guests

[patchbot@xxxxxxx]

commit 3113e12ead7866e6f400b67504f6aa77f3bc6582
Author:     Jan Beulich <JBeulich@xxxxxxxx>
AuthorDate: Mon May 9 12:57:02 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon May 9 12:57:02 2016 +0200

    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@xxxxxxxx>
    Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: d22c9bf7c3067b17cbd9cdfd8b81941dd6fb8d77
    master date: 2016-04-28 15:06:56 +0200
---
 xen/arch/x86/time.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 7a01c90..d191321 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1757,7 +1757,12 @@ custom_param("tsc", tsc_parse);
 u64 gtime_to_gtsc(struct domain *d, u64 time)
 {
     if ( !is_hvm_domain(d) )
-        time = max_t(s64, time - d->arch.vtsc_offset, 0);
+    {
+        if ( time < d->arch.vtsc_offset )
+            return -scale_delta(d->arch.vtsc_offset - time,
+                                &d->arch.ns_to_vtsc);
+        time -= d->arch.vtsc_offset;
+    }
     return scale_delta(time, &d->arch.ns_to_vtsc);
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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