[XEN PATCH v2] x86/hpet: Disable legacy replacement mode after IRQ test

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

As far as I understand the HPET legacy mode is not required after the
timer IRQ test. For previous discussion see [1] and [2]. Keeping it
enabled prevents reaching deeper C-states on some systems and thereby
also S0ix residency. So disable it after the timer IRQ test worked. Note
that this code path is only reached when opt_hpet_legacy_replacement < 0,
so explicit user choice is still honored.

Link: 
https://lore.kernel.org/xen-devel/cb408368-077d-edb5-b4ad-f80086db48c1@xxxxxxxxxxxxxxxxxxxxxx/
 # [1]
Link: 
https://lore.kernel.org/xen-devel/20230718122603.2002-1-simon@xxxxxxxxxxxxxxxxxxxxxx/
 # [2]
Signed-off-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
---

Changes in v2:

 - Always disable legacy mode after test, not only when ARAT is
   available. See [3] for reasoning.

[3]: 
https://lore.kernel.org/xen-devel/ac77ecba-6804-1d16-60dc-f184e5d31dcb@xxxxxxxxxxxxxxxxxxxxxx/

---
 xen/arch/x86/io_apic.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index 041233b9b7..b4b4cd5939 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -1967,6 +1967,8 @@ static void __init check_timer(void)
 
             if ( timer_irq_works() )
             {
+                printk(XENLOG_INFO "IRQ test with HPET Legacy Replacement Mode 
worked. Disabling it again.\n");
+                hpet_disable_legacy_replacement_mode();
                 local_irq_restore(flags);
                 return;
             }
-- 
2.40.1