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>
master commit: 82713ec8d2b65d17f13e46a131e38bfe5baf8bd6
master date: 2014-04-22 12:07:37 +0200

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index c4da848c1dcd5569366d424184c54c7af27d1008..cbdc58a5649a779fa50f37c354687f6862239555 100644 (file)
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -732,7 +732,7 @@ 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;

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 7358551b56f6c212ad8917ef69ffa9124e1370f2..7b6d55797c8e2a693b6ce6837a1619e7c4baae69 100644 (file)
--- a/xen/arch/x86/time.c
+++ b/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, cyclone, acpi. */
@@ -1922,10 +1923,16 @@ 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() &&
+             ((is_hvm_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);