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

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Date:   Wed, 09 Dec 2020 19:54:26 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 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 guess that timer_irq_works() should instead do:
>
> 	local_irq_save(flags);
> 	local_irq_enable();
> 	...
> 	local_irq_disable();
> 	local_irq_restore(flags);
>
> ... assuming we consider that legitimate?

Nah. That's old and insane gunk.

Thanks,

        tglx
---
 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();
 }
 
 /*