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

From: "Jürgen Groß" <jgross@suse.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	"VMware, Inc." <pv-drivers@vmware.com>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use popf
Date: Mon, 23 Nov 2020 06:21:56 +0100	[thread overview]
Message-ID: <894a4cec-8426-4152-d391-474711040c5b@suse.com> (raw)
In-Reply-To: <CALCETrXOGhXoOJpzhAMqD7iibi09WzbGk9SWVH7JzA=d5uarWA@mail.gmail.com>

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

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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