Commit c445535c authored by Ani Sinha's avatar Ani Sinha Committed by Wei Liu
Browse files

x86/hyperv: fix for unwanted manipulation of sched_clock when TSC marked unstable 



Marking TSC as unstable has a side effect of marking sched_clock as
unstable when TSC is still being used as the sched_clock. This is not
desirable. Hyper-V ultimately uses a paravirtualized clock source that
provides a stable scheduler clock even on systems without TscInvariant
CPU capability. Hence, mark_tsc_unstable() call should be called _after_
scheduler clock has been changed to the paravirtualized clocksource. This
will prevent any unwanted manipulation of the sched_clock. Only TSC will
be correctly marked as unstable.

Signed-off-by: default avatarAni Sinha <ani@anisinha.ca>
Reviewed-by: default avatarMichael Kelley <mikelley@microsoft.com>
Tested-by: default avatarMichael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20210713030522.1714803-1-ani@anisinha.ca


Signed-off-by: default avatarWei Liu <wei.liu@kernel.org>
parent ba3f5839

arch/x86/kernel/cpu/mshyperv.c

+7 −2
Original line number Diff line number Diff line
@@ -375,8 +375,6 @@ static void __init ms_hyperv_init_platform(void) 
	if (ms_hyperv.features & HV_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");

	}



	/*
@@ -437,6 +435,13 @@ static void __init ms_hyperv_init_platform(void) 
	/* Register Hyper-V specific clocksource */

	hv_init_clocksource();

#endif

	/*

	 * TSC should be marked as unstable only after Hyper-V

	 * clocksource has been initialized. This ensures that the

	 * stability of the sched_clock is not altered.

	 */

	if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))

		mark_tsc_unstable("running on Hyper-V");

}



static bool __init ms_hyperv_x2apic_available(void)