Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use popf

[Jürgen Groß via Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

On 22.11.20 22:44, Andy Lutomirski wrote:
> On Sat, Nov 21, 2020 at 10:55 PM Jürgen Groß <jgross@xxxxxxxx> 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
> }

This couldn't be a WARN_ON_ONCE() then (or it would be a catch all).
Another approach might be to open-code the WARN_ON_ONCE(), like:

#ifdef CONFIG_DEBUG_ENTRY
extern void warn_bogus_irqrestore(bool *once);
#endif

static __always_inline void arch_local_irq_restore(unsigned long flags)
{
	if (!arch_irqs_disabled_flags(flags))
		arch_local_irq_enable();
#ifdef CONFIG_DEBUG_ENTRY
	{
		static bool once;

		if (unlikely(arch_local_irq_save() & X86_EFLAGS_IF))
			warn_bogus_irqrestore(&once);
	}
#endif
}


Juergen

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