linux-kernel - [PATCH v2 2/2] x86/hyper-v: Enable PIT shutdown quirk

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Date:   Sun, 4 Nov 2018 03:48:57 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "jgross@...e.com" <jgross@...e.com>,
        "akataria@...are.com" <akataria@...are.com>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        vkuznets <vkuznets@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        KY Srinivasan <kys@...rosoft.com>
CC:     Michael Kelley <mikelley@...rosoft.com>
Subject: [PATCH v2 2/2] x86/hyper-v: Enable PIT shutdown quirk

Hyper-V emulation of the PIT has a quirk such that the normal
PIT shutdown path doesn't work. Enable the PIT code that
handles this quirk.

Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
---
 arch/x86/kernel/cpu/mshyperv.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 1c72f38..65b2f88 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -20,6 +20,7 @@
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/kexec.h>
+#include <linux/i8253.h>
 #include <asm/processor.h>
 #include <asm/hypervisor.h>
 #include <asm/hyperv-tlfs.h>
@@ -295,6 +296,16 @@ static void __init ms_hyperv_init_platform(void)
 	if (efi_enabled(EFI_BOOT))
 		x86_platform.get_nmi_reason = hv_get_nmi_reason;
 
+	/*
+	 * Hyper-V VMs have a PIT emulation quirk such that zeroing the
+	 * counter register during PIT shutdown restarts the PIT. So it
+	 * continues to interrupt @18.2 HZ. Setting i8253_clear_counter
+	 * to false tells pit_shutdown() not to zero the counter so that
+	 * the PIT really is shutdown. Generation 2 VMs don't have a PIT,
+	 * and setting this value has no effect.
+	 */
+	i8253_clear_counter = false;
+
 #if IS_ENABLED(CONFIG_HYPERV)
 	/*
 	 * Setup the hook to get control post apic initialization.
-- 
1.8.3.1