Set SysTick CLKSOURCE bit before enabling SysTick

Author: jefftenney

portable/ARMv8M/non_secure/port.c

@@ -620,8 +620,12 @@ __attribute__( ( weak ) ) void vPortSetupTimerInterrupt( void ) /* PRIVILEGED_FU
     }
     #endif /* configUSE_TICKLESS_IDLE */
 
-    /* Stop and reset the SysTick. */
-    portNVIC_SYSTICK_CTRL_REG = 0UL;
+    /* Stop and reset the SysTick. Set the CLKSOURCE bit now while the SysTick
+     * is stopped to accommodate a bug in QEMU versions older than 7.0.0. The
+     * code that starts Systick (below) sets or clears the CLKSOURCE to match
+     * the build configuration, so this temporary setting of the CLKSOURCE bit
+     * has no impact on the actual operation of the SysTick. */
+    portNVIC_SYSTICK_CTRL_REG = portNVIC_SYSTICK_CLK_BIT;
     portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;
 
     /* Configure SysTick to interrupt at the requested rate. */