[PATCH 1/2] x86/hyperv: Allow guests to enable InvariantTSC

[Andrea Parri]

If the hardware supports TSC scaling, Hyper-V will set bit 15 of the
HV_PARTITION_PRIVILEGE_MASK in guest VMs with a compatible Hyper-V
configuration version.  Bit 15 corresponds to the
AccessTscInvariantControls privilege.  If this privilege bit is set,
guests can access the HvSyntheticInvariantTscControl MSR: guests can
set bit 0 of this synthetic MSR to enable the InvariantTSC feature.
After setting the synthetic MSR, CPUID will enumerate support for
InvariantTSC.

Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
---
 arch/x86/include/asm/hyperv-tlfs.h | 5 +++++
 arch/x86/kernel/cpu/mshyperv.c     | 7 ++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 7741e211f7f51..5f10f7f2098db 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h
@@ -86,6 +86,8 @@
 #define HV_X64_ACCESS_FREQUENCY_MSRS		BIT(11)
 /* AccessReenlightenmentControls privilege */
 #define HV_X64_ACCESS_REENLIGHTENMENT		BIT(13)
+/* AccessTscInvariantControls privilege */
+#define HV_X64_ACCESS_TSC_INVARIANT		BIT(15)
 
 /*
  * Feature identification: indicates which flags were specified at partition
@@ -278,6 +280,9 @@
 #define HV_X64_MSR_TSC_EMULATION_CONTROL	0x40000107
 #define HV_X64_MSR_TSC_EMULATION_STATUS		0x40000108
 
+/* TSC invariant control */
+#define HV_X64_MSR_TSC_INVARIANT_CONTROL	0x40000118
+
 /*
  * Declare the MSR used to setup pages used to communicate with the hypervisor.
  */
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 267daad8c0360..105844d542e5c 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -286,7 +286,12 @@ static void __init ms_hyperv_init_platform(void)
 	machine_ops.shutdown = hv_machine_shutdown;
 	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
 #endif
-	mark_tsc_unstable("running on Hyper-V");
+	if (ms_hyperv.features & HV_X64_ACCESS_TSC_INVARIANT) {
+		wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
+		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
+	} else {
+		mark_tsc_unstable("running on Hyper-V");
+	}
 
 	/*
 	 * Generation 2 instances don't support reading the NMI status from
-- 
2.23.0