Linux Virtualization - Sep 2020 - [PATCH RFC v1 05/18] clocksource/hyperv: use MSR-based access if running as root

Wei Liu <wei.liu at kernel.org> writes:
> Signed-off-by: Wei Liu <wei.liu at kernel.org>
> ---
>  drivers/clocksource/hyperv_timer.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/clocksource/hyperv_timer.c
b/drivers/clocksource/hyperv_timer.c
> index 09aa44cb8a91..fe96082ce85e 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -426,6 +426,9 @@ static bool __init hv_init_tsc_clocksource(void)
>  	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
>  		return false;
>  
> +	if (hv_root_partition)
> +		return false;
> +
Out of pure curiosity,

TSC page clocksource seems to be available to the root partition (as
HV_MSR_REFERENCE_TSC_AVAILABLE is set), why don't we use it? (I
understand that with TSC scaling support in modern CPUs even migration
is a no-issue and we can use raw TSC but this all seems to be
independent from root/non-root partition question).
>  	hv_read_reference_counter = read_hv_clock_tsc;
>  	phys_addr = virt_to_phys(hv_get_tsc_page());
-- 
Vitaly