Re: [tip:x86/urgent] x86: Fix bogus warning in apic_noop.apic_write()

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 7 Dec 2009, Cyrill Gorcunov wrote:
> On Mon, Dec 07, 2009 at 04:48:51PM +0100, Thomas Gleixner wrote:
> > On Mon, 7 Dec 2009, Cyrill Gorcunov wrote:
> > > On Mon, Dec 07, 2009 at 12:18:37PM +0000, tip-bot for Thomas Gleixner wrote:
> > > > Commit-ID:  a946d8f11f0da9cfc714248036fcfd3a794d1e27
> > > > Gitweb:     http://git.kernel.org/tip/a946d8f11f0da9cfc714248036fcfd3a794d1e27
> > > > Author:     Thomas Gleixner <tglx@linutronix.de>
> > > > AuthorDate: Mon, 7 Dec 2009 12:59:46 +0100
> > > > Committer:  Ingo Molnar <mingo@elte.hu>
> > > > CommitDate: Mon, 7 Dec 2009 13:16:37 +0100
> > > > 
> > > > x86: Fix bogus warning in apic_noop.apic_write()
> > > > 
> > > > apic_noop is used to provide dummy apic functions. It's installed
> > > > when the CPU has no APIC or when the APIC is disabled on the kernel
> > > > command line.
> > > > 
> > > > The apic_noop implementation of apic_write() warns when the CPU has
> > > > an APIC or when the APIC is not disabled.
> > > > 
> > > > That's bogus. The warning should only happen when the CPU has an
> > > > APIC _AND_ the APIC is not disabled. apic_noop.apic_read() has the
> > > > correct check.
> > > > 
> > > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > > > Cc: Cyrill Gorcunov <gorcunov@openvz.org>
> > > > Cc: <stable@kernel.org> # in <= .32 this typo resides in native_apic_write_dummy()
> > > > LKML-Reference: <alpine.LFD.2.00.0912071255420.3089@localhost.localdomain>
> > > > Signed-off-by: Ingo Molnar <mingo@elte.hu>
> > > > ---
> > > >  arch/x86/kernel/apic/apic_noop.c |    2 +-
> > > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > ...
> > > 
> > > Hi Thomas, Ingo,
> > > 
> > > please do not change it. There are still machines without
> > > cpu_has_apic bit support so with this patch any attempt
> > > to write to 82489DX will success. So the former code has
> > > been using "OR" by a purpose, there is no error.
> > 
> > Err, your warning has the following false positive:
> > 
> >      cpu_has_apic == true and disable_apic == true
> 
> This combination impossible at moment (by "impossible" I mean
> at moment of apic_write action). When apic disabled via boot
> option cpu_has_apic cleared as well.
> 
> static int __init setup_disableapic(char *arg)
> {
> 	disable_apic = 1;
> 	setup_clear_cpu_cap(X86_FEATURE_APIC);
> 	return 0;
> }
> 
> And, btw if some code is trying to write to apic when
> it's disabled via boot option -- it means the code is
> buggy and this is not a false positive but rather proper
> warning.
> 
> Thomas, if you've changed this code I suppose you saw some
> warning triggered, right? Could you pointed me on it?

http://www.kerneloops.org/searchweek.php?search=native_apic_write_dummy