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

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Message-Id: <1359121036-16309-1-git-send-email-olaf@aepfle.de>
Date:	Fri, 25 Jan 2013 14:37:16 +0100
From:	Olaf Hering <olaf@...fle.de>
To:	linux-kernel@...r.kernel.org
Cc:	Olaf Hering <olaf@...fle.de>, stable@...nel.org,
	KY Srinivasan <kys@...rosoft.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: [PATCH] x86: Hyper-V: register clocksource only if its advertised

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>
Cc: stable@...nel.org
Cc: KY Srinivasan <kys@...rosoft.com>
Cc: Greg KH <gregkh@...uxfoundation.org>
---
 arch/x86/kernel/cpu/mshyperv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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 = {
-- 
1.8.1.1

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