linux-kernel - Re: [RFC PATCH] x86/kvm: do not touch watchdogs in pvclock

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Date:   Sat, 17 Jul 2021 09:24:43 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        "Paul E . McKenney" <paulmck@...nel.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] x86/kvm: do not touch watchdogs in pvclock

On (21/07/16 14:34), Sergey Senozhatsky wrote:
> 
> <IRQ>
> apic_timer_interrupt()
>  smp_apic_timer_interrupt()
>   hrtimer_interrupt()
>    _raw_spin_lock_irqsave()
>     lock_acquire()
>      __lock_acquire()
>       sched_clock_cpu()
>        sched_clock()
>         kvm_sched_clock_read()
>          kvm_clock_read()
>           pvclock_clocksource_read()
>            pvclock_touch_watchdogs()
> 
> Since this is VM and VCPU resume path, jiffies still maybe
> be outdated here, which is often the case on my device.
> pvclock_clocksource_read() clears PVCLOCK_GUEST_STOPPED,
> touches watchdogs, but it uses stale jiffies: 4294740764
> (for example).

Hmm, on the other hand, there is probably nothing that guarantees
that the first watchdog hard IRQ we execute on a resuming VCPU is
going to see updated jiffies, it still can use stale jiffies.