linux-kernel - [tip:x86/hyperv] x86/hyperv: Read TSC frequency from a synthetic MSR

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Message-ID: <tip-71c2a2d0a81f096a2932fccb39a500116fece554@git.kernel.org>
Date:   Thu, 22 Jun 2017 06:40:56 -0700
From:   tip-bot for Vitaly Kuznetsov <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, jloeser@...rosoft.com, hpa@...or.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        vkuznets@...hat.com, kys@...rosoft.com, lprosek@...hat.com,
        tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: [tip:x86/hyperv] x86/hyperv: Read TSC frequency from a synthetic
 MSR

Commit-ID:  71c2a2d0a81f096a2932fccb39a500116fece554
Gitweb:     http://git.kernel.org/tip/71c2a2d0a81f096a2932fccb39a500116fece554
Author:     Vitaly Kuznetsov <vkuznets@...hat.com>
AuthorDate: Thu, 22 Jun 2017 18:07:30 +0800
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 22 Jun 2017 15:35:12 +0200

x86/hyperv: Read TSC frequency from a synthetic MSR

It was found that SMI_TRESHOLD of 50000 is not enough for Hyper-V
guests in nested environment and falling back to counting jiffies
is not an option for Gen2 guests as they don't have PIT. As Hyper-V
provides TSC frequency in a synthetic MSR we can just use this information
instead of doing a error prone calibration.

Reported-and-tested-by: Ladi Prosek <lprosek@...hat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Stephen Hemminger <sthemmin@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Jork Loeser <jloeser@...rosoft.com>
Cc: devel@...uxdriverproject.org
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
Link: http://lkml.kernel.org/r/20170622100730.18112-3-vkuznets@redhat.com

---
 arch/x86/kernel/cpu/mshyperv.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 3563c8e..70e717f 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -161,6 +161,15 @@ static int hv_nmi_unknown(unsigned int val, struct pt_regs *regs)
 }
 #endif
 
+static unsigned long hv_get_tsc_khz(void)
+{
+	unsigned long freq;
+
+	rdmsrl(HV_X64_MSR_TSC_FREQUENCY, freq);
+
+	return freq / 1000;
+}
+
 static void __init ms_hyperv_init_platform(void)
 {
 	int hv_host_info_eax;
@@ -193,6 +202,12 @@ static void __init ms_hyperv_init_platform(void)
 			hv_host_info_edx >> 24, hv_host_info_edx & 0xFFFFFF);
 	}
 
+	if (ms_hyperv.features & HV_X64_ACCESS_FREQUENCY_MSRS &&
+	    ms_hyperv.misc_features & HV_FEATURE_FREQUENCY_MSRS_AVAILABLE) {
+		x86_platform.calibrate_tsc = hv_get_tsc_khz;
+		x86_platform.calibrate_cpu = hv_get_tsc_khz;
+	}
+
 #ifdef CONFIG_X86_LOCAL_APIC
 	if (ms_hyperv.features & HV_X64_ACCESS_FREQUENCY_MSRS &&
 	    ms_hyperv.misc_features & HV_FEATURE_FREQUENCY_MSRS_AVAILABLE) {