x86/time: don't use virtual TSC if host and guest frequencies are equal

Commit 82713ec8d2 ("x86: use native RDTSC(P) execution when guest and
host frequencies are the same") left out optimization for PV guests
when host and guest run at the same frequency.

For such a case we should be able not to use virtual TSC regardless
of whether we are runing before or after a migration (i.e. regardless
of incarnation value).

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
[jb: retain parts of the original comment]
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/time.c

@@ -2024,17 +2024,18 @@ void tsc_set_info(struct domain *d,
         d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
         d->arch.tsc_khz = gtsc_khz ?: cpu_khz;
         set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000);
+
         /*
-         * In default mode use native TSC if the host has safe TSC and:
-         *  HVM/PVH: host and guest frequencies are the same (either
-         *           "naturally" or via TSC scaling)
-         *  PV: guest has not migrated yet (and thus arch.tsc_khz == cpu_khz)
+         * In default mode use native TSC if the host has safe TSC and
+         * host and guest frequencies are the same (either "naturally" or
+         * - for HVM/PVH - via TSC scaling).
+         * When a guest is created, gtsc_khz is passed in as zero, making
+         * d->arch.tsc_khz == cpu_khz. Thus no need to check incarnation.
          */
         if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() &&
-             (is_hvm_domain(d) ?
-              (d->arch.tsc_khz == cpu_khz ||
-               hvm_get_tsc_scaling_ratio(d->arch.tsc_khz)) :
-              incarnation == 0) )
+             (d->arch.tsc_khz == cpu_khz ||
+              (is_hvm_domain(d) &&
+               hvm_get_tsc_scaling_ratio(d->arch.tsc_khz))) )
         {
     case TSC_MODE_NEVER_EMULATE:
             d->arch.vtsc = 0;