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

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Message-ID: <20201120115943.GD3021@hirez.programming.kicks-ass.net>
Date:   Fri, 20 Nov 2020 12:59:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, luto@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use
 popf

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?