X86: Prefer TSC-deadline timer in Xen

The new TSC Deadline Timer offers system software a low overhead
per-logical-thread deadline timer in TSC units.

The timer is implemented via a new architectural 64-bit register,
IA32_TSC_DEADLINE_MSR. Reads and writes of this MSR occur in program
order, but are non-serializing.

The support for this feature is indicated by
CPUID.01H:ECX.TSC_Deadline[bit 24] =3D 1 as documented in the Intel
Architecture Software Developer's Manual.

The LOCAL APIC on new processors has a mode where its underlying
hardware timer can now be accessed via the non-serializing
IA32_TSC_DEADLINE_MSR in TSC tick units.

If this mode is present, prefer it over the traditional LAPIC timer
mode.  KERN_DEBUG dmesg will print "TSC deadline timer enabled" when
TDT is used.

Bootparam "tdt=off" is available to revert to LAPIC timer mode.

This patch is based on original work by Len Brown for Linux kernel.

cc: Len Brown <len.brown@intel.com>
Signed-off-by: Wei Gang <gang.wei@intel.com>
Signed-off-by: Keir Fraser <keir@xen.org>

1 files changed, 10 insertions, 8 deletions

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 0564c398b7..ad37cb3075 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -662,26 +662,28 @@ static void __init init_platform_timer(void)
            freq_string(pts->frequency), pts->name);
 }
 
-void cstate_restore_tsc(void)
+u64 stime2tsc(s_time_t stime)
 {
     struct cpu_time *t;
     struct time_scale sys_to_tsc;
     s_time_t stime_delta;
-    u64 new_tsc;
-
-    if ( boot_cpu_has(X86_FEATURE_NONSTOP_TSC) )
-        return;
 
     t = &this_cpu(cpu_time);
     sys_to_tsc = scale_reciprocal(t->tsc_scale);
 
-    stime_delta = read_platform_stime() - t->stime_master_stamp;
+    stime_delta = stime - t->stime_local_stamp;
     if ( stime_delta < 0 )
         stime_delta = 0;
 
-    new_tsc = t->local_tsc_stamp + scale_delta(stime_delta, &sys_to_tsc);
+    return t->local_tsc_stamp + scale_delta(stime_delta, &sys_to_tsc);
+}
+
+void cstate_restore_tsc(void)
+{
+    if ( boot_cpu_has(X86_FEATURE_NONSTOP_TSC) )
+        return;
 
-    write_tsc(new_tsc);
+    write_tsc(stime2tsc(read_platform_stime()));
 }
 
 /***************************************************************************