[PATCH 1/5] x86/time: deal with negative deltas in get_s_time_fixed()

[Jan Beulich <jbeulich@xxxxxxxx>]

Callers may pass in TSC values from before the local TSC stamp was last
updated (this would in particular be the case when the TSC was latched, a
time rendezvous occurs, and the latched value is used only afterwards).
scale_delta(), otoh, deals with unsigned values, and hence would treat
negative incoming deltas as huge positive values, yielding entirely bogus
return values.

Fixes: 88e64cb785c1 ("x86/HVM: use fixed TSC value when saving or restoring 
domain")
Reported-by: Антон Марков <akmarkov45@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
An alternative might be to have scale_delta() deal with signed deltas, yet
that seemed more risky to me.

There could likely be more Fixes: tags; the one used is the oldest
applicable one, from what I can tell.

A similar issue looks to exist in read_xen_timer() and its read_cycle()
helper, if we're scheduled out (and beck in) between reading of the TSC
and calculation of the delta (involving ->tsc_timestamp). Am I
overlooking anything there?

stime2tsc() guards against negative deltas by using 0 instead; I'm not
quite sure that's correct either.

amd_check_erratum_1474() (next to its call to tsc_ticks2ns()) has a
comment towards the TSC being "sane", but is that correct? Due to
TSC_ADJUST, rdtsc() may well return a huge value (and the TSC would then
wrap through 0 at some point). Shouldn't we subtract boot_tsc_stamp before
calling tsc_ticks2ns()?

A similar issue looks to exist in tsc_get_info(), again when rdtsc()
possibly returns a huge value due to TSC_ADJUST. Once again I wonder
whether we shouldn't subtract boot_tsc_stamp.

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1649,8 +1649,13 @@ s_time_t get_s_time_fixed(u64 at_tsc)
         tsc = at_tsc;
     else
         tsc = rdtsc_ordered();
-    delta = tsc - t->stamp.local_tsc;
-    return t->stamp.local_stime + scale_delta(delta, &t->tsc_scale);
+
+    if ( tsc >= t->stamp.local_tsc )
+        delta = scale_delta(tsc - t->stamp.local_tsc, &t->tsc_scale);
+    else
+        delta = -scale_delta(t->stamp.local_tsc - tsc, &t->tsc_scale);
+
+    return t->stamp.local_stime + delta;
 }
 
 s_time_t get_s_time(void)