[PATCH 2/2] xen/x86: ioapic: Bail out from timer_irq_works() as soon as possible

[Julien Grall <julien@xen.org>]

From: Julien Grall <jgrall@amazon.com>

Currently timer_irq_works() will wait the full 100ms before checking
that pit0_ticks has been incremented at least 4 times.

However, the bulk of the BIOS/platform should not have a buggy timer.
So waiting for the full 100ms is a bit harsh.

Rework the logic to only wait until 100ms passed or we saw more than
4 ticks. So now, in the good case, this will reduce the wait time
to ~50ms.

Signed-off-by: Julien Grall <jgrall@amazon.com>
---
 xen/arch/x86/io_apic.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index 4875bb97003f..0bd774962a68 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -1509,6 +1509,8 @@ static void __init setup_ioapic_ids_from_mpc(void)
 static int __init timer_irq_works(void)
 {
     unsigned long t1, flags;
+    /* Wait for maximum 10 ticks */
+    unsigned long msec = (10 * 1000) / HZ;
 
     if ( no_timer_check )
         return 1;
@@ -1517,19 +1519,25 @@ static int __init timer_irq_works(void)
 
     local_save_flags(flags);
     local_irq_enable();
-    /* Let ten ticks pass... */
-    mdelay((10 * 1000) / HZ);
-    local_irq_restore(flags);
 
-    /*
-     * Expect a few ticks at least, to be sure some possible
-     * glue logic does not lock up after one or two first
-     * ticks in a non-ExtINT mode.  Also the local APIC
-     * might have cached one ExtINT interrupt.  Finally, at
-     * least one tick may be lost due to delays.
-     */
-    if ( (ACCESS_ONCE(pit0_ticks) - t1) > 4 )
+    while ( msec-- )
+    {
+        mdelay(1);
+        /*
+         * Expect a few ticks at least, to be sure some possible
+         * glue logic does not lock up after one or two first
+         * ticks in a non-ExtINT mode.  Also the local APIC
+         * might have cached one ExtINT interrupt.  Finally, at
+         * least one tick may be lost due to delays.
+         */
+        if ( (ACCESS_ONCE(pit0_ticks) - t1) <= 4 )
+            continue;
+
+        local_irq_restore(flags);
         return 1;
+    }
+
+    local_irq_restore(flags);
 
     return 0;
 }
-- 
2.40.1