[PATCH v2 1/2] xen/x86: io_apic: Introduce a command line option to skip timer check

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

Currently, Xen will spend ~100ms to check if the timer works. If the
Admin knows their platform have a working timer, then it would be
handy to be able to bypass the check.

Introduce a command line option 'pit-irq-works' for this purpose.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

---

Changelog since v1:
    - Rename the command line option. I went with pit-irq-works rather
      than timer-irq-works because Roger thought it would be better suited
    - Rework the command line description
---
 docs/misc/xen-command-line.pandoc | 11 +++++++++++
 xen/arch/x86/io_apic.c            | 11 +++++++++++
 2 files changed, 22 insertions(+)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 8e65f8bd18bf..c382b061b302 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2535,6 +2535,17 @@ pages) must also be specified via the tbuf_size 
parameter.
 ### tickle_one_idle_cpu
 > `= <boolean>`
 
+### pit-irq-works (x86)
+> `=<boolean>`
+
+> Default: `false`
+
+Disables the code which tests for broken timer IRQ sources. Enabling
+this option will reduce boot time on HW where the timer works properly.
+
+If the system is unstable when enabling the option, then it means you
+may have a broken HW and therefore the testing cannot be be skipped.
+
 ### timer_slop
 > `= <integer>`
 
diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index d11c880544e6..238b6c1c2837 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -57,6 +57,14 @@ bool __initdata ioapic_ack_forced;
 int __read_mostly nr_ioapic_entries[MAX_IO_APICS];
 int __read_mostly nr_ioapics;
 
+/*
+ * The logic to check if the timer is working is expensive. So allow
+ * the admin to bypass it if they know their platform doesn't have
+ * a buggy timer.
+ */
+static bool __initdata pit_irq_works;
+boolean_param("pit-irq-works", pit_irq_works);
+
 /*
  * Rough estimation of how many shared IRQs there are, can
  * be changed anytime.
@@ -1502,6 +1510,9 @@ static int __init timer_irq_works(void)
 {
     unsigned long t1, flags;
 
+    if ( pit_irq_works )
+        return 1;
+
     t1 = ACCESS_ONCE(pit0_ticks);
 
     local_save_flags(flags);
-- 
2.40.1