Re: [PATCH] Correct system time calculation

[Owen Smith <owen.smith@xxxxxxxxxx>]

Interestingly, xen.h defines the time calculation in a comment, but does not 
account for tsc_shift being a signed int8_t value.

https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/xen.h;h=82b9c05a76b7faedded8778fb8274a0d3d5d31e4;hb=HEAD#l677

Reviewed-by: Owen Smith <owen.smith@xxxxxxxxxx>

________________________________________
From: win-pv-devel <win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx> on behalf of Tu 
Dinh <ngoc-tu.dinh@xxxxxxxxxx>
Sent: 10 November 2025 7:13 PM
To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Tu Dinh
Subject: [PATCH] Correct system time calculation

SharedInfoGetTime uses the naive formula ((Tsc << TscShift) *
TscSystemMul) >> 32 to calculate the current system time. However, there
are two issues: firstly, TscShift may be negative (to indicate a right
shift); and secondly, multiplying the shifted Tsc with TscSystemMul will
throw away the result's upper 64 bits.

Adjust for negative shift amounts and use UnsignedMultiplyExtract128 to
correctly perform the multiplication.

Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
---
 src/xenbus/shared_info.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/xenbus/shared_info.c b/src/xenbus/shared_info.c
index 06e0aec..fe29470 100644
--- a/src/xenbus/shared_info.c
+++ b/src/xenbus/shared_info.c
@@ -356,6 +356,7 @@ SharedInfoGetTime(
     ULONGLONG                       NanoSeconds;
     ULONGLONG                       Timestamp;
     ULONGLONG                       Tsc;
+    ULONGLONG                       Age;
     ULONGLONG                       SystemTime;
     ULONG                           TscSystemMul;
     CHAR                            TscShift;
@@ -404,10 +405,15 @@ SharedInfoGetTime(
     KeLowerIrql(Irql);

     // Number of elapsed ticks since timestamp was captured
-    Tsc -= Timestamp;
+    Age = Tsc - Timestamp;
+    if (TscShift < 0)
+        Age >>= -TscShift;
+    else
+        Age <<= TscShift;
+    Age = UnsignedMultiplyExtract128(Age, TscSystemMul, 32);

     // Time in nanoseconds since boot
-    SystemTime += ((Tsc << TscShift) * TscSystemMul) >> 32;
+    SystemTime += Age;

     Trace("WALLCLOCK TIME AT BOOT: Seconds = %llu NanoSeconds = %llu\n",
           Seconds,
--
2.51.0.windows.2



--
Ngoc Tu Dinh | Vates XCP-ng Developer

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