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Peter Anvin" , xen-devel , Thomas Gleixner , Boris Ostrovsky Subject: Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use popf Message-ID: <20201209132710.GA8566@C02TD0UTHF1T.local> References: <20201120114630.13552-1-jgross@suse.com> <20201120114630.13552-6-jgross@suse.com> <20201120115943.GD3021@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Sun, Nov 22, 2020 at 01:44:53PM -0800, Andy Lutomirski wrote: > On Sat, Nov 21, 2020 at 10:55 PM Jürgen Groß 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 > } I was just talking to Peter on IRC about implementing the same thing for arm64, so could we put this in the generic irqflags code? IIUC we can use raw_irqs_disabled() to do the check. As this isn't really entry specific (and IIUC the cases this should catch would break lockdep today), maybe we should add a new DEBUG_IRQFLAGS for this, that DEBUG_LOCKDEP can also select? Something like: #define local_irq_restore(flags) \ do { \ if (!raw_irqs_disabled_flags(flags)) { \ trace_hardirqs_on(); \ } else if (IS_ENABLED(CONFIG_DEBUG_IRQFLAGS) { \ if (unlikely(raw_irqs_disabled()) \ warn_bogus_irqrestore(); \ } \ raw_local_irq_restore(flags); \ } while (0) ... perhaps? (ignoring however we deal with once-ness). Thanks, Mark.