[Xen-changelog] [xen staging] x86/viridian: drop a wrong invalid value from reference TSC implementation

[patchbot@xxxxxxx]

commit c9d9b5f4c9dbc077f2a601f6cd20c075cbbb3cf6
Author:     Wei Liu <liuwe@xxxxxxxxxxxxx>
AuthorDate: Fri Dec 20 21:08:28 2019 +0000
Commit:     Wei Liu <wl@xxxxxxx>
CommitDate: Mon Dec 23 10:51:29 2019 +0000

    x86/viridian: drop a wrong invalid value from reference TSC implementation
    
    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@xxxxxxxxxxxxx>
    Reviewed-by: Paul Durrant <paul@xxxxxxx>
---
 xen/arch/x86/hvm/viridian/time.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/hvm/viridian/time.c b/xen/arch/x86/hvm/viridian/time.c
index 6b2d745f3a..0f1cd9e208 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;
 
@@ -77,10 +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 */
-        seq = 1;
-
-    p->tsc_sequence = seq;
+    p->tsc_sequence = seq ? seq : 1; /* Avoid 'invalid' value 0 */
 }
 
 static uint64_t trc_val(const struct domain *d, int64_t offset)
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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