x86: use native RDTSC(P) execution when guest and host frequencies are the same

We should be able to continue using native RDTSC(P) execution on
HVM/PVH guests after migration if host and guest frequencies are
equal (this includes the case when the frequencies are made equal
by TSC scaling feature).

This also allows us to revert main part of commit 4aab59a3 (svm: Do not
intercept RDTSC(P) when TSC scaling is supported by hardware) which
was wrong: while RDTSC intercepts were disabled domain's vtsc could
still be set, leading to inconsistent view of guest's TSC.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/svm/svm.c

@@ -728,7 +728,7 @@ static void svm_set_rdtsc_exiting(struct vcpu *v, bool_t enable)
     general1_intercepts &= ~GENERAL1_INTERCEPT_RDTSC;
     general2_intercepts &= ~GENERAL2_INTERCEPT_RDTSCP;
 
-    if ( enable && !cpu_has_tsc_ratio )
+    if ( enable )
     {
         general1_intercepts |= GENERAL1_INTERCEPT_RDTSC;
         general2_intercepts |= GENERAL2_INTERCEPT_RDTSCP;

xen/arch/x86/time.c

@@ -37,6 +37,7 @@
 #include <asm/hpet.h>
 #include <io_ports.h>
 #include <asm/setup.h> /* for early_time_init */
+#include <asm/hvm/svm/svm.h> /* for cpu_has_tsc_ratio */
 #include <public/arch-x86/cpuid.h>
 
 /* opt_clocksource: Force clocksource to one of: pit, hpet, acpi. */
@@ -1889,10 +1890,16 @@ void tsc_set_info(struct domain *d,
         d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
         d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
         set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
-        /* use native TSC if initial host has safe TSC, has not migrated
-         * yet and tsc_khz == cpu_khz */
-        if ( host_tsc_is_safe() && incarnation == 0 &&
-                d->arch.tsc_khz == cpu_khz )
+        /*
+         * 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)
+         */
+        if ( host_tsc_is_safe() &&
+             ((has_hvm_container_domain(d) &&
+               (d->arch.tsc_khz == cpu_khz || cpu_has_tsc_ratio)) ||
+              incarnation == 0) )
             d->arch.vtsc = 0;
         else 
             d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);