linux-kernel - Re: [PATCH v2 05/12] x86: rework arch_local_irq

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Date:   Sun, 22 Nov 2020 13:44:53 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Jürgen Groß <jgross@...e.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "VMware, Inc." <pv-drivers@...are.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use popf

On Sat, Nov 21, 2020 at 10:55 PM Jürgen Groß <jgross@...e.com> wrote:
>
> On 20.11.20 12:59, Peter Zijlstra wrote:
> > On Fri, Nov 20, 2020 at 12:46:23PM +0100, Juergen Gross wrote:
> >> +static __always_inline void arch_local_irq_restore(unsigned long flags)
> >> +{
> >> +    if (!arch_irqs_disabled_flags(flags))
> >> +            arch_local_irq_enable();
> >> +}
> >
> > If someone were to write horrible code like:
> >
> >       local_irq_disable();
> >       local_irq_save(flags);
> >       local_irq_enable();
> >       local_irq_restore(flags);
> >
> > we'd be up some creek without a paddle... now I don't _think_ we have
> > genius code like that, but I'd feel saver if we can haz an assertion in
> > there somewhere...
> >
> > Maybe something like:
> >
> > #ifdef CONFIG_DEBUG_ENTRY // for lack of something saner
> >       WARN_ON_ONCE((arch_local_save_flags() ^ flags) & X86_EFLAGS_IF);
> > #endif
> >
> > At the end?
>
> I'd like to, but using WARN_ON_ONCE() in include/asm/irqflags.h sounds
> like a perfect receipt for include dependency hell.
>
> We could use a plain asm("ud2") instead.

How about out-of-lining it:

#ifdef CONFIG_DEBUG_ENTRY
extern void warn_bogus_irqrestore();
#endif

static __always_inline void arch_local_irq_restore(unsigned long flags)
{
       if (!arch_irqs_disabled_flags(flags)) {
               arch_local_irq_enable();
       } else {
#ifdef CONFIG_DEBUG_ENTRY
               if (unlikely(arch_local_irq_save() & X86_EFLAGS_IF))
                    warn_bogus_irqrestore();
#endif
}