[PATCH v4 1/3] x86: Use native RDTSC(P) execution when guest and host frequencies are the same

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

We should be able to continue using native RDTSC(P) execution 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>
---
 xen/arch/x86/hvm/svm/svm.c |    2 +-
 xen/arch/x86/time.c        |   13 +++++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 4fd5376..813e775 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/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;
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index f904af2..b84d127 100644
--- 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, acpi. */
@@ -1889,10 +1890,14 @@ 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 initial host has safe TSC and either has not
+         * migrated yet or tsc_khz == cpu_khz (either "naturally" or via
+         * TSC scaling)
+         */
+        if ( host_tsc_is_safe() &&
+             (incarnation == 0 || d->arch.tsc_khz == cpu_khz ||
+              cpu_has_tsc_ratio) )
             d->arch.vtsc = 0;
         else 
             d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);
-- 
1.7.1