[xen staging-4.21] x86/time: use native TSC scaling factors when TSC is not scaled

[patchbot@xxxxxxx]

commit 27748a734393c1c6ec44aacacc6736749fbe72bf
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Apr 20 12:28:50 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 20 12:28:50 2026 +0200

    x86/time: use native TSC scaling factors when TSC is not scaled
    
    When running HVM guest in native TSC mode avoid using the recalculated vTSC
    scaling factors based on the cpu_khz value.  Using the kHz based frequency
    leads to the TSC scaling values possibly not being the same as the ones
    used by the per CPU cpu_time->tsc_scale field, which introduces skew
    between the guest and Xen's calculations of the system time.
    
    On a 2gHz system, where the frequency is possibly detected as 1999999999Hz
    (note this is a worse-case scenario), the cpu_khz variable will be set to
    1999999kHz, and hence 999Hz cycles will be not accounted for per second.
    Over a second (the time synchronization period), this leads to a skew of:
    
    cycles * 1 / (Hz freq) = 999 / 1999999999 = 499,5ns
    
    So far this has gone unnoticed because the time synchronization rendezvous
    forces the update of the tsc_timestamp and system_time fields in the vCPU
    time info area, and hence the skew only accumulates up to the rendezvous
    period.  Attempting to remove the rendezvous causes the skew to grow
    unbounded.
    
    Fix by using the native TSC scaling values (as used by Xen) when the guest
    TSC is not scaled.
    
    Fixes: eab8a90be723 ("x86/time: scale host TSC in pvclock properly")
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 305c6b901a44aa6e16d3bf7d104d7c08331d1731
    master date: 2026-04-16 15:28:32 +0200
---
 xen/arch/x86/time.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index bb36038e0c..d9f7772ab4 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1702,17 +1702,25 @@ static void collect_time_info(const struct vcpu *v,
     else
     {
         if ( is_hvm_domain(d) && hvm_tsc_scaling_supported )
-        {
             tsc_stamp            = hvm_scale_tsc(d, t->stamp.local_tsc);
-            u->tsc_to_system_mul = d->arch.vtsc_to_ns.mul_frac;
-            u->tsc_shift         = d->arch.vtsc_to_ns.shift;
-        }
         else
-        {
             tsc_stamp            = t->stamp.local_tsc;
+
+        /*
+         * HVM guests using the native TSC ratio should use the same per-CPU
+         * scaling factors as Xen.  This ensures time keeping is always in sync
+         * between Xen and the guest.
+         */
+        if ( tsc_stamp == t->stamp.local_tsc )
+        {
             u->tsc_to_system_mul = t->tsc_scale.mul_frac;
             u->tsc_shift         = t->tsc_scale.shift;
         }
+        else
+        {
+            u->tsc_to_system_mul = d->arch.vtsc_to_ns.mul_frac;
+            u->tsc_shift         = d->arch.vtsc_to_ns.shift;
+        }
     }
 
     u->tsc_timestamp = tsc_stamp;
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.21