linux-kernel - Re: [BUG] msr-trace.h:42 suspicious rcu_dereference

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Date:   Tue, 29 Nov 2016 15:04:31 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Andi Kleen <andi@...stfloor.org>,
        Jan Stancek <jstancek@...hat.com>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

On Tue, Nov 29, 2016 at 02:16:49PM +0100, Borislav Petkov wrote:
> On Mon, Nov 21, 2016 at 05:06:54PM +0100, Borislav Petkov wrote:
> > IOW, what's the worst thing that can happen if we did this below?
> > 
> > We basically get rid of the detection and switch the timer to broadcast
> > mode immediately on the halting CPU.
> > 
> > amd_e400_idle() is behind an "if (cpu_has_bug(c, X86_BUG_AMD_APIC_C1E))"
> > check so it will run on the affected CPUs only...
> > 
> > Thoughts?
> 
> Actually, here's a better version. The E400 detection works only after
> ACPI has been enabled so we piggyback the end of acpi_init().
> 
> We don't need the MSR read now - we do
> 
> 	if (static_cpu_has_bug(X86_BUG_AMD_APIC_C1E))
> 
> on the idle path which is as fast as it gets.
> 
> Any complaints about this before I go and test it everywhere?
> 
> It builds and boots in my guest here ok, not that it means a whole lot.
> 
> The good news is, I have collected a fleet of boxes which all have that
> erratum so testing should be pretty reliable. Something that doesn't
> happen everytime!

+1 one test server over here.. will check ;-)

jirka