linux-kernel - [PATCH 1/3] x86: Hyper-V: register clocksource only if its advertised

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Message-Id: <1359507108-26091-1-git-send-email-kys@microsoft.com>
Date:	Tue, 29 Jan 2013 16:51:46 -0800
From:	"K. Y. Srinivasan" <kys@...rosoft.com>
To:	x86@...nel.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	olaf@...fle.de, apw@...onical.com, jasowang@...hat.com,
	tglx@...utronix.de, hpa@...or.com, JBeulich@...e.com, bp@...en8.de
Cc:	Olaf Hering <"[mailto:olaf@...fle.de]"@linuxonhyperv.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>, stable@...nel.org
Subject: [PATCH 1/3] x86: Hyper-V: register clocksource only if its advertised

From: Olaf Hering <[mailto:olaf@...fle.de]>

Enable hyperv_clocksource only if its advertised as a feature.
XenServer 6 returns the signature which is checked in
ms_hyperv_platform(), but it does not offer all features. Currently the
clocksource is enabled unconditionally in ms_hyperv_init_platform(), and
the result is a hanging guest.

Hyper-V spec Bit 1 indicates the availability of Partition Reference
Counter.  Register the clocksource only if this bit is set.

The guest in question prints this in dmesg:
 [    0.000000] Hypervisor detected: Microsoft HyperV
 [    0.000000] HyperV: features 0x70, hints 0x0

This bug can be reproduced easily be setting 'viridian=1' in a HVM domU
.cfg file. A workaround without this patch is to boot the HVM guest with
'clocksource=jiffies'.

Signed-off-by: Olaf Hering <olaf@...fle.de>
Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
Cc: stable@...nel.org
Cc: Greg KH <gregkh@...uxfoundation.org>
---
 arch/x86/kernel/cpu/mshyperv.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 0a630dd..646d192 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -68,7 +68,8 @@ static void __init ms_hyperv_init_platform(void)
 	printk(KERN_INFO "HyperV: features 0x%x, hints 0x%x\n",
 	       ms_hyperv.features, ms_hyperv.hints);
 
-	clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100);
+	if (ms_hyperv.features & HV_X64_MSR_TIME_REF_COUNT_AVAILABLE)
+		clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100);
 }
 
 const __refconst struct hypervisor_x86 x86_hyper_ms_hyperv = {
-- 
1.7.4.1

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