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

[Mark Rutland <mark.rutland@xxxxxxx>]

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@xxxxxxx>

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();
>  }
>  
>  /*