Message-ID: <m1obss39a4.fsf@fess.ebiederm.org>
Date:	Tue, 21 Feb 2012 00:01:07 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Don Zickus <dzickus@...hat.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, linux-kernel@...r.kernel.org,
	mingo@...hat.com, hpa@...or.com, torvalds@...ux-foundation.org,
	kexec@...ts.infradead.org, vgoyal@...hat.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/debug] x86/kdump: No need to disable ioapic/ lapic in crash path

Don Zickus <dzickus@...hat.com> writes:

2> On Fri, Feb 17, 2012 at 07:21:52PM -0800, Eric W. Biederman wrote:
>> Don Zickus <dzickus@...hat.com> writes:
>> 
>> > On Fri, Feb 17, 2012 at 04:41:01AM -0800, Eric W. Biederman wrote:
>> >> 
>> >> The fix with a guarantee of no more scope creep is to just disable the
>> >> nmi watchdog on the kexec on panic path.
>> >> 
>> >> Don if you have time please figure out is needed to ignore nmi's and
>> >> possible record and/or report them while we boot, otherwise please cook
>> >> up a patch that just disables the nmi watchdog wherever we are sending
>> >> it from (the local apic or the ioapic).
>> >
>> > Can I keep things even simpler?  The original problem was the deadlock
>> > with the ioapic lock.  We fixed that by removing the call to
>> > disable_IO_APIC().  Can we just leave the disable_local_APIC calls in
>> > there for now?  Is there any real harm?
>> 
>> > All this rewrite is going to take time which will delay fixing a current
>> > problem with kexec on panic, the ioapic deadlock.
>> 
>> Hmm.
>> 
>> My apologies I just realized that we can not disable the nmi watchdog
>> safely in all cases.  To avoid the deadlock we fundamentally can not
>> write to the io_apic, because the locks are the io_apic write path.
>> The nmi watchdog can be sourced from either the local apics or the
>> io_apics.  To disable the nmi_watchdog we need at least potentially
>> to write io_apic.
>
> I am curious where you see the nmi watchdog being sourced from the ioapic?
> I thought I removed that code 3 or 4 releases ago.

In my memory, and in references to the code in comments in various apic
related code.  I couldn't figure out what the current code was doing and
assumed the implementation was equivalent.  It does look like you
removed the code that used the io_apic.  I still haven't figured
out just how the new implementation works yet.

So maybe in the short term we can safely just stomp the timer that
triggers the nmi watchdog in the local apic.  Over the long term that
feels like it is just asking for trouble.

I wonder if the reason that we have an hpet stomp in that code is
for a similar reason.  Did we ever source nmi's from the hpet timer?

>> So it appears to me that the only reasonable and robust thing we can
>> do is to ignore nmis in the kexec on panic path.
>> 
>> So it looks to me that the only path forward at this point is to fix
>> the other bug where an unexpected nmi will kill the kexec on panic boot.
>> 
>> I just took a look at the code in /sbin/kexec and that code does not in
>> fact change the idt except when we switch to 16bit mode, which we
>> definitely do not do in the kexec on panic case.  So it appears that we
>> don't need to coordinate an /sbin/kexec release with a kernel release to
>> ignore nmis.
>> 
>> In fact it looks like we only need to fix the interrupt descriptors
>> loaded in machine_kexec_64.c and head64.c to ignore nmis.
>> 
>> At which point we will have fixed two bugs and have a much more reliable
>> kexec on panic implementation.
>
> Ok.  I'll talk with Vivek about how the can be implemented.

Thanks.  It really doesn't look very hard.  Just a tiny idt with
an nmi entry that says iret.

Eric
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