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

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

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();
 }
 
 /*