linux-kernel - Re: [patch V2 03/17] x86/traps: Remove pointless irq enable from do_spurious_interrupt

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Date:   Wed, 23 Oct 2019 17:49:52 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-arch@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [patch V2 03/17] x86/traps: Remove pointless irq enable from
 do_spurious_interrupt_bug()

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@...utronix.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.

-- 
Josh