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[PATCH v2 2/2] hyper-v: read TSC frequency from a synthetic MSR

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  [PATCH v2 2/2] hyper-v: read TSC frequency from a synthetic MSR Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-06-22 12:10 +0200
    [tip:x86/hyperv] x86/hyperv: Read TSC frequency from a synthetic  MSR tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-06-22 15:50 +0200

#1672487 — [PATCH v2 2/2] hyper-v: 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-by: Ladi Prosek <lprosek@redhat.com>
Tested-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Vitaly Kuznetsov <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..200c521 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) {
-- 
2.9.4

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#1672642 — [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@redhat.com>
AuthorDate: Thu, 22 Jun 2017 18:07:30 +0800
Committer:  Thomas Gleixner <tglx@linutronix.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@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jork Loeser <jloeser@microsoft.com>
Cc: devel@linuxdriverproject.org
Cc: "K. Y. Srinivasan" <kys@microsoft.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) {

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