linux-kernel - Re: [v2 PATCH 4/4] x86/kvm: Add guest side support for virtual suspend time injection

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Message-ID: <87lf59qp1p.ffs@tglx>
Date:   Tue, 10 Aug 2021 17:48:02 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Hikaru Nishida <hikalium@...omium.org>,
        linux-kernel@...r.kernel.org, dme@....org, mlevitsk@...hat.com
Cc:     suleiman@...gle.com, Hikaru Nishida <hikalium@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        John Stultz <john.stultz@...aro.org>,
        Juergen Gross <jgross@...e.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Mike Travis <mike.travis@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>, kvm@...r.kernel.org,
        x86@...nel.org
Subject: Re: [v2 PATCH 4/4] x86/kvm: Add guest side support for virtual
 suspend time injection

On Fri, Aug 06 2021 at 19:07, Hikaru Nishida wrote:
>  arch/x86/Kconfig                    | 13 ++++++++++
>  arch/x86/include/asm/idtentry.h     |  4 +++
>  arch/x86/include/asm/kvm_para.h     |  9 +++++++
>  arch/x86/kernel/kvmclock.c          | 40 +++++++++++++++++++++++++++++
>  include/linux/timekeeper_internal.h |  4 +++
>  kernel/time/timekeeping.c           | 33 ++++++++++++++++++++++++

Again, this wants to be split into infrastructure and usage.

> --- a/include/linux/timekeeper_internal.h
> +++ b/include/linux/timekeeper_internal.h
> @@ -124,6 +124,10 @@ struct timekeeper {
> 	u32			ntp_err_mult;
> 	/* Flag used to avoid updating NTP twice with same second */
> 	u32			skip_second_overflow;
> +#ifdef CONFIG_KVM_VIRT_SUSPEND_TIMING_GUEST
> +	/* suspend_time_injected keeps the duration injected through kvm */
> +	u64			suspend_time_injected;

This is KVM only, so please can we have a name for that struct member
which reflects this?

> +#endif
>  #ifdef CONFIG_DEBUG_TIMEKEEPING
> 	long			last_warning;
> 	/*

> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 3ac3fb479981..424c61d38646 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -2125,6 +2125,39 @@ static u64 logarithmic_accumulation(struct timekeeper *tk, u64 offset,
>  	return offset;
>  }
>  
> +#ifdef CONFIG_KVM_VIRT_SUSPEND_TIMING_GUEST
> +/*
> + * timekeeping_inject_virtual_suspend_time - Inject virtual suspend time
> + * when requested by the kvm host.

If this is an attempt to provide a kernel-doc comment for this function,
then it's clearly a failed attempt and aside of that malformatted.

> + * This function should be called under irq context.

Why? There is no reason for being called from interrupt context and
nothing inforces it.

> + */
> +void timekeeping_inject_virtual_suspend_time(void)
> +{
> +	/*
> +	 * Only updates shadow_timekeeper so the change will be reflected
> +	 * on the next call of timekeeping_advance().

No. That's broken.

    timekeeping_inject_virtual_suspend_time();

    do_settimeofday() or do_adjtimex()

       timekeeping_update(tk, TK_MIRROR...);

and your change to the shadow timekeeper is gone.

Of course there is also no justification for this approach. What's wrong
with updating it right away?

> +	 */
> +	struct timekeeper *tk = &shadow_timekeeper;
> +	unsigned long flags;
> +	struct timespec64 delta;
> +	u64 suspend_time;

Please sort variables in reverse fir tree order and not randomly as you
see fit.

> +
> +	raw_spin_lock_irqsave(&timekeeper_lock, flags);
> +	suspend_time = kvm_get_suspend_time();
> +	if (suspend_time > tk->suspend_time_injected) {
> +		/*
> +		 * Do injection only if the time is not injected yet.
> +		 * suspend_time and tk->suspend_time_injected values are
> +		 * cummrative, so take a diff and inject the duration.

cummrative?

> +		 */
> +		delta = ns_to_timespec64(suspend_time - tk->suspend_time_injected);
> +		__timekeeping_inject_sleeptime(tk, &delta);
> +		tk->suspend_time_injected = suspend_time;

It's absolutely unclear how this storage and diff magic works and the
comment is not helping someone not familiar with the implementation of
kvm_get_suspend_time() and the related code at all. Please explain
non-obvious logic properly.

Thanks,

        tglx