Re: [patch V2 03/17] x86/traps: Remove pointless irq enable from do_spurious_interrupt_bug()

From: Thomas Gleixner <tglx@linutronix.de>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>, Will Deacon <will@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-arch@vger.kernel.org,
	Mike Rapoport <rppt@linux.ibm.com>,
	Miroslav Benes <mbenes@suse.cz>
Subject: Re: [patch V2 03/17] x86/traps: Remove pointless irq enable from do_spurious_interrupt_bug()
Date: Thu, 24 Oct 2019 01:18:55 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1910240117030.1852@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20191023224952.d73mataiisu3u4tg@treble>

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On Wed, 23 Oct 2019, Josh Poimboeuf wrote:
> On Thu, Oct 24, 2019 at 12:35:27AM +0200, Thomas Gleixner wrote:
> > On Wed, 23 Oct 2019, Josh Poimboeuf wrote:
> > 
> > > On Wed, Oct 23, 2019 at 02:27:08PM +0200, Thomas Gleixner wrote:
> > > > That function returns immediately after conditionally reenabling interrupts which
> > > > is more than pointless and requires the ASM code to disable interrupts again.
> > > > 
> > > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > > > ---
> > > >  arch/x86/kernel/traps.c |    1 -
> > > >  1 file changed, 1 deletion(-)
> > > > 
> > > > --- a/arch/x86/kernel/traps.c
> > > > +++ b/arch/x86/kernel/traps.c
> > > > @@ -871,7 +871,6 @@ do_simd_coprocessor_error(struct pt_regs
> > > >  dotraplinkage void
> > > >  do_spurious_interrupt_bug(struct pt_regs *regs, long error_code)
> > > >  {
> > > > -	cond_local_irq_enable(regs);
> > > >  }
> > > 
> > > I think we can just remove this handler altogether.  The Intel and AMD
> > > manuals say vector 15 (X86_TRAP_SPURIOUS) is reserved.
> > 
> > Right, but this has history. Pentium Pro Erratum:
> > 
> >   PROBLEM: If the APIC subsystem is configured in mixed mode with Virtual
> >   Wire mode implemented through the local APIC, an interrupt vector of 0Fh
> >   (Intel reserved encoding) may be generated by the local APIC (Int 15).
> >   This vector may be generated upon receipt of a spurious interrupt (an
> >   interrupt which is removed before the system receives the INTA sequence)
> >   instead of the programmed 8259 spurious interrupt vector.
> > 
> >   IMPLICATION: The spurious interrupt vector programmed in the 8259 is
> >   normally handled by an operating system’s spurious interrupt
> >   handler. However, a vector of 0Fh is unknown to some operating systems,
> >   which would crash if this erratum occurred.
> > 
> > Initially (2.1.) there was a printk() in that handler, which later got
> > ifdeffed out (2.1.54).
> > 
> > So I rather keep that thing at least as long as we support PPro :) Even if
> > we ditch that the handler is not really hurting anyone.
> 
> Ah.  I guess we could remove the idtentry for 64-bit then?  Anyway the
> above would be a good comment for the function.

That doesn't buy much. Who knows how many other CPUs issue vector 15
occasionally and will then crash and burn. Better safe than sorry :)

Thanks,

	tglx