linux-kernel - Re: [patch 1/5] tracehook: Provide TIF_NOTIFY

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Message-Id: <05C28F0C-5D8C-4A77-8D1B-FAC91DCF1115@amacapital.net>
Date:   Thu, 1 Aug 2019 10:02:38 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julia Cartwright <julia@...com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, kvm@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [patch 1/5] tracehook: Provide TIF_NOTIFY_RESUME handling for KVM


> On Aug 1, 2019, at 7:48 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Thu, Aug 01, 2019 at 04:32:51PM +0200, Thomas Gleixner wrote:
>> +#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
>> +/**
>> + * tracehook_handle_notify_resume - Notify resume handling for virt
>> + *
>> + * Called with interrupts and preemption enabled from VMENTER/EXIT.
>> + */
>> +void tracehook_handle_notify_resume(void)
>> +{
>> +    local_irq_disable();
>> +    while (test_and_clear_thread_flag(TIF_NOTIFY_RESUME)) {
>> +        local_irq_enable();
>> +        tracehook_notify_resume(NULL);
>> +        local_irq_disable();
>> +    }
>> +    local_irq_enable();
> 
> I'm confused by the IRQ state swizzling here, what is it doing?

Me too. Also, why is a loop needed?

> 
>> +}
>> +EXPORT_SYMBOL_GPL(tracehook_handle_notify_resume);
>> +#endif
>> 
>>