linux-kernel - Re: [PATCH] x86/hotplug: Silence APIC only after all irq's are migrated

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Message-ID: <20200820195811.GA22799@otc-nc-03>
Date:   Thu, 20 Aug 2020 12:58:11 -0700
From:   "Raj, Ashok" <ashok.raj@...el.com>
To:     Evan Green <evgreen@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sukumar Ghorai <sukumar.ghorai@...el.com>,
        Srikanth Nandamuri <srikanth.nandamuri@...el.com>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, stable@...r.kernel.org,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH] x86/hotplug: Silence APIC only after all irq's are
 migrated

On Thu, Aug 20, 2020 at 11:21:24AM -0700, Evan Green wrote:
> > >
> > > I'm slightly unclear about whether interrupts are disabled at the core
> > > by this point or not. I followed native_cpu_disable() up to
> > > __cpu_disable(), up to take_cpu_down(). This is passed into a call to
> > > stop_machine_cpuslocked(), where interrupts get disabled at the core.
> > > So unless there's another path, it seems like interrupts are always
> > > disabled at the core by this point.
> >
> > local_irq_disable() just does cli() which allows interrupts to trickle
> > in to the IRR bits, and once you do sti() things would flow back for
> > normal interrupt processing.
> >
> >
> > >
> > > If interrupts are always disabled, then the comment above is a little
> >
> > Disable interrupts is different from disabling LAPIC. Once you do the
> > apic_soft_disable(), there is nothing flowing into the LAPIC except
> > for INIT, NMI, SMI and SIPI messages.
> >
> > This turns off the pipe for all other interrupts to enter LAPIC. Which
> > is different from doing a cli().
> 
> I understand the distinction. I was mostly musing on the difference in
> behavior your change causes if this function is entered with
> interrupts enabled at the core (ie sti()). But I think it never is, so
> that thought is moot.
> 
> I could never repro the issue reliably on comet lake after Thomas'
> original fix. But I can still repro it easily on jasper lake. And this
> patch fixes the issue for me on that platform. Thanks for the fix.
> 
> Reviewed-by: Evan Green <evgreen@...omium.org>
> Tested-by: Evan Green <evgreen@...omium.org>

Thanks Evan for testing. I'll wait for thomas if he finds anything else
that needs to be fixed and send a final v2 after fixing the typos and
others identified by Randy. 

Cheers,
Ashok