Re: [PATCH] clocksource: hyper-v: Use lapic timer in a TDX VM without paravisor

[Roman Kisel <romank@linux.microsoft.com>]

On 6/18/2024 5:25 PM, Dexuan Cui wrote:
> In a TDX VM without paravisor, currently the default timer is the Hyper-V
> timer, which depends on the slow VM Reference Counter MSR: the Hyper-V TSC
> page is not enabled in such a VM because the VM uses Invariant TSC as a
> better clocksource and it's challenging to mark the Hyper-V TSC page shared
> in very early boot.
> 
> Lower the rating of the Hyper-V timer so the local APIC timer becomes the
> the default timer in such a VM. This change should cause no perceivable
> performance difference.
> 
> Cc: stable@vger.kernel.org # 6.6+
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>   arch/x86/kernel/cpu/mshyperv.c     |  6 +++++-
>   drivers/clocksource/hyperv_timer.c | 16 +++++++++++++++-
>   2 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index e0fd57a8ba840..745af47ca0459 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -449,9 +449,13 @@ static void __init ms_hyperv_init_platform(void)
>   			ms_hyperv.hints &= ~HV_X64_APIC_ACCESS_RECOMMENDED;
>   
>   			if (!ms_hyperv.paravisor_present) {
> -				/* To be supported: more work is required.  */
> +				/* Use Invariant TSC as a better clocksource. */
>   				ms_hyperv.features &= ~HV_MSR_REFERENCE_TSC_AVAILABLE;
>   
> +				/* Use the Ref Counter in case Invariant TSC is unavailable. */
> +				if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
> +					pr_warn("Hyper-V: Invariant TSC is unavailable\n");
> +
>   				/* HV_MSR_CRASH_CTL is unsupported. */
>   				ms_hyperv.misc_features &= ~HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;
>   
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index b2a080647e413..99177835cadec 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -137,7 +137,21 @@ static int hv_stimer_init(unsigned int cpu)
>   	ce->name = "Hyper-V clockevent";
>   	ce->features = CLOCK_EVT_FEAT_ONESHOT;
>   	ce->cpumask = cpumask_of(cpu);
> -	ce->rating = 1000;
> +
> +	/*
> +	 * Lower the rating of the Hyper-V timer in a TDX VM without paravisor,
> +	 * so the local APIC timer (lapic_clockevent) is the default timer in
> +	 * such a VM. The Hyper-V timer is not preferred in such a VM because
> +	 * it depends on the slow VM Reference Counter MSR (the Hyper-V TSC
> +	 * page is not enbled in such a VM because the VM uses Invariant TSC
> +	 * as a better clocksource and it's challenging to mark the Hyper-V
> +	 * TSC page shared in very early boot).
> +	 */
> +	if (!ms_hyperv.paravisor_present && hv_isolation_type_tdx())
> +		ce->rating = 90;
> +	else
> +		ce->rating = 1000;
> +
>   	ce->set_state_shutdown = hv_ce_shutdown;
>   	ce->set_state_oneshot = hv_ce_set_oneshot;
>   	ce->set_next_event = hv_ce_set_next_event;

LGTM.

Reviewed-by: Roman Kisel <romank@linux.microsoft.com>

-- 
Thank you,
Roman