[PATCH v2] x86/hyperv: add comment describing TSC_INVARIANT_CONTROL MSR setting bit 0

[Ani Sinha]

Commit dce7cd62754b5 ("x86/hyperv: Allow guests to enable InvariantTSC")
added the support for HV_X64_MSR_TSC_INVARIANT_CONTROL. Setting bit 0
of this synthetic MSR will allow hyper-v guests to report invariant TSC
CPU feature through CPUID. This comment adds this explanation to the code
and mentions where the Intel's generic platform init code reads this
feature bit from CPUID. The comment will help developers understand how
the two parts of the initialization (hyperV specific and non-hyperV
specific generic hw init) are related.

Signed-off-by: Ani Sinha <ani@xxxxxxxxxxx>
---
 arch/x86/kernel/cpu/mshyperv.c | 9 +++++++++
 1 file changed, 9 insertions(+)

changelog:
v1: initial patch
v2: slight comment update based on received feedback.

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 715458b7729a..3b05dab3086e 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -368,6 +368,15 @@ static void __init ms_hyperv_init_platform(void)
 	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
 #endif
 	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
+		/*
+		 * Writing to synthetic MSR 0x40000118 updates/changes the
+		 * guest visible CPUIDs. Setting bit 0 of this MSR  enables
+		 * guests to report invariant TSC feature through CPUID
+		 * instruction, CPUID 0x800000007/EDX, bit 8. See code in
+		 * early_init_intel() where this bit is examined. The
+		 * setting of this MSR bit should happen before init_intel()
+		 * is called.
+		 */
 		wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
 		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
 	}
-- 
2.25.1