[Xen-devel] [PATCH 5/4] x86/viridian: drop a wrong invalid value from reference TSC implementation

[Wei Liu <wl@xen.org>]

The only invalid value mentioned in Hyper-V TLFS 5.0c is 0. Michael
Kelley confirmed that 0xFFFFFFFF was never used [0].

[0] https://lists.xen.org/archives/html/xen-devel/2019-12/msg01564.html

Signed-off-by: Wei Liu <liuwe@microsoft.com>
---
 xen/arch/x86/hvm/viridian/time.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/hvm/viridian/time.c b/xen/arch/x86/hvm/viridian/time.c
index 6b2d745f3a..b8280a1a60 100644
--- a/xen/arch/x86/hvm/viridian/time.c
+++ b/xen/arch/x86/hvm/viridian/time.c
@@ -45,14 +45,9 @@ static void update_reference_tsc(const struct domain *d, bool initialize)
     if ( !host_tsc_is_safe() || d->arch.vtsc )
     {
         /*
-         * The specification states that valid values of TscSequence range
-         * from 0 to 0xFFFFFFFE. The value 0xFFFFFFFF is used to indicate
-         * this mechanism is no longer a reliable source of time and that
-         * the VM should fall back to a different source.
-         *
-         * Server 2012 (6.2 kernel) and 2012 R2 (6.3 kernel) actually
-         * violate the spec. and rely on a value of 0 to indicate that this
-         * enlightenment should no longer be used.
+         * The value 0 is used to indicate this mechanism is no longer a
+         * reliable source of time and that the VM should fall back to a
+         * different source.
          */
         p->tsc_sequence = 0;
 
@@ -77,7 +72,7 @@ static void update_reference_tsc(const struct domain *d, bool initialize)
     smp_wmb();
 
     seq = p->tsc_sequence + 1;
-    if ( seq == 0xFFFFFFFF || seq == 0 ) /* Avoid both 'invalid' values */
+    if ( seq == 0 ) /* Avoid 'invalid' value */
         seq = 1;
 
     p->tsc_sequence = seq;
-- 
2.20.1


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