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

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Date:   Sun, 22 Nov 2020 07:55:55 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        "VMware, Inc." <pv-drivers@...are.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        luto@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        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 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.


Juergen

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