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

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Message-ID: <20201210111008.GB88655@C02TD0UTHF1T.local>
Date:   Thu, 10 Dec 2020 11:10:08 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, luto@...nel.org,
        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 Wed, Dec 09, 2020 at 07:54:26PM +0100, Thomas Gleixner wrote:
> On Wed, Dec 09 2020 at 18:15, Mark Rutland wrote:
> > In arch/x86/kernel/apic/io_apic.c's timer_irq_works() we do:
> >
> > 	local_irq_save(flags);
> > 	local_irq_enable();
> >
> > 	[ trigger an IRQ here ]
> >
> > 	local_irq_restore(flags);
> >
> > ... and in check_timer() we call that a number of times after either a
> > local_irq_save() or local_irq_disable(), eventually trailing with a
> > local_irq_disable() that will balance things up before calling
> > local_irq_restore().

I gave the patchlet below a spin with my debug patch, and it boots
cleanly for me under QEMU. If you spin it as a real patch, feel free to
add:

Tested-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  arch/x86/kernel/apic/io_apic.c |   22 ++++++----------------
>  1 file changed, 6 insertions(+), 16 deletions(-)
> 
> --- a/arch/x86/kernel/apic/io_apic.c
> +++ b/arch/x86/kernel/apic/io_apic.c
> @@ -1618,21 +1618,16 @@ static void __init delay_without_tsc(voi
>  static int __init timer_irq_works(void)
>  {
>  	unsigned long t1 = jiffies;
> -	unsigned long flags;
>  
>  	if (no_timer_check)
>  		return 1;
>  
> -	local_save_flags(flags);
>  	local_irq_enable();
> -
>  	if (boot_cpu_has(X86_FEATURE_TSC))
>  		delay_with_tsc();
>  	else
>  		delay_without_tsc();
>  
> -	local_irq_restore(flags);
> -
>  	/*
>  	 * Expect a few ticks at least, to be sure some possible
>  	 * glue logic does not lock up after one or two first
> @@ -1641,10 +1636,10 @@ static int __init timer_irq_works(void)
>  	 * least one tick may be lost due to delays.
>  	 */
>  
> -	/* jiffies wrap? */
> -	if (time_after(jiffies, t1 + 4))
> -		return 1;
> -	return 0;
> +	local_irq_disable();
> +
> +	/* Did jiffies advance? */
> +	return time_after(jiffies, t1 + 4);
>  }
>  
>  /*
> @@ -2117,13 +2112,12 @@ static inline void __init check_timer(vo
>  	struct irq_cfg *cfg = irqd_cfg(irq_data);
>  	int node = cpu_to_node(0);
>  	int apic1, pin1, apic2, pin2;
> -	unsigned long flags;
>  	int no_pin1 = 0;
>  
>  	if (!global_clock_event)
>  		return;
>  
> -	local_irq_save(flags);
> +	local_irq_disable();
>  
>  	/*
>  	 * get/set the timer IRQ vector:
> @@ -2191,7 +2185,6 @@ static inline void __init check_timer(vo
>  			goto out;
>  		}
>  		panic_if_irq_remap("timer doesn't work through Interrupt-remapped IO-APIC");
> -		local_irq_disable();
>  		clear_IO_APIC_pin(apic1, pin1);
>  		if (!no_pin1)
>  			apic_printk(APIC_QUIET, KERN_ERR "..MP-BIOS bug: "
> @@ -2215,7 +2208,6 @@ static inline void __init check_timer(vo
>  		/*
>  		 * Cleanup, just in case ...
>  		 */
> -		local_irq_disable();
>  		legacy_pic->mask(0);
>  		clear_IO_APIC_pin(apic2, pin2);
>  		apic_printk(APIC_QUIET, KERN_INFO "....... failed.\n");
> @@ -2232,7 +2224,6 @@ static inline void __init check_timer(vo
>  		apic_printk(APIC_QUIET, KERN_INFO "..... works.\n");
>  		goto out;
>  	}
> -	local_irq_disable();
>  	legacy_pic->mask(0);
>  	apic_write(APIC_LVT0, APIC_LVT_MASKED | APIC_DM_FIXED | cfg->vector);
>  	apic_printk(APIC_QUIET, KERN_INFO "..... failed.\n");
> @@ -2251,7 +2242,6 @@ static inline void __init check_timer(vo
>  		apic_printk(APIC_QUIET, KERN_INFO "..... works.\n");
>  		goto out;
>  	}
> -	local_irq_disable();
>  	apic_printk(APIC_QUIET, KERN_INFO "..... failed :(.\n");
>  	if (apic_is_x2apic_enabled())
>  		apic_printk(APIC_QUIET, KERN_INFO
> @@ -2260,7 +2250,7 @@ static inline void __init check_timer(vo
>  	panic("IO-APIC + timer doesn't work!  Boot with apic=debug and send a "
>  		"report.  Then try booting with the 'noapic' option.\n");
>  out:
> -	local_irq_restore(flags);
> +	local_irq_enable();
>  }
>  
>  /*