linux-kernel - Re: [EXTERNAL] [PATCH 2/2] KVM: x86: disable interrupts while pvclock_gtod_sync

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Date:   Thu, 01 Apr 2021 16:27:29 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     mtosatti@...hat.com, vkuznets@...hat.com,
        syzbot+b282b65c2c68492df769@...kaller.appspotmail.com
Subject: Re: [EXTERNAL] [PATCH 2/2] KVM: x86: disable interrupts while
 pvclock_gtod_sync_lock is taken

On Tue, 2021-03-30 at 12:59 -0400, Paolo Bonzini wrote:
> @@ -2686,13 +2688,13 @@ static int kvm_guest_time_update(struct
> kvm_vcpu *v)
>          * If the host uses TSC clock, then passthrough TSC as stable
>          * to the guest.
>          */
> -       spin_lock(&ka->pvclock_gtod_sync_lock);
> +       spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
>         use_master_clock = ka->use_master_clock;
>         if (use_master_clock) {
>                 host_tsc = ka->master_cycle_now;
>                 kernel_ns = ka->master_kernel_ns;
>         }
> -       spin_unlock(&ka->pvclock_gtod_sync_lock);
> +       spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
> 
>         /* Keep irq disabled to prevent changes to the clock */
>         local_irq_save(flags);

That seems a little gratuitous at the end; restoring the flags as part
of the spin_unlock_irqrestore() and then immediately calling
local_irq_save().

Is something going to complain if we just use spin_unlock() there and
then later restore the flags with the existing local_irq_restore()?

Or should we move the local_irq_save() up above the existing
spin_lock() and leave the spin lock/unlock as they are?

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