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Message-ID: <1b37ba872b4d2e6186f8e172b95c36d92153d952.camel@infradead.org>
Subject: Re: [EXTERNAL] [PATCH 2/2] KVM: x86: disable interrupts while
 pvclock_gtod_sync_lock is taken
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     mtosatti@redhat.com, vkuznets@redhat.com,
        syzbot+b282b65c2c68492df769@syzkaller.appspotmail.com
Date:   Thu, 01 Apr 2021 16:27:29 +0100
In-Reply-To: <20210330165958.3094759-3-pbonzini@redhat.com>
References: <20210330165958.3094759-1-pbonzini@redhat.com>
         <20210330165958.3094759-3-pbonzini@redhat.com>
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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 =3D ka->use_master_clock;
>         if (use_master_clock) {
>                 host_tsc =3D ka->master_cycle_now;
>                 kernel_ns =3D ka->master_kernel_ns;
>         }
> -       spin_unlock(&ka->pvclock_gtod_sync_lock);
> +       spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
>=20
>         /* 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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