Re: [PATCH] x86/i8259: Skip probing when ACPI/MADT advertises PCAT compatibility

[Hans de Goede]

Hi,

On 10/25/23 23:04, Thomas Gleixner wrote:
> David and a few others reported that on certain newer systems some legacy
> interrupts fail to work correctly.
> 
> Debugging revealed that the BIOS of these systems leaves the legacy PIC in
> uninitialized state which makes the PIC detection fail and the kernel
> switches to a dummy implementation.
> 
> Unfortunately this fallback causes quite some code to fail as it depends on
> checks for the number of legacy PIC interrupts or the availability of the
> real PIC.
> 
> In theory there is no reason to use the PIC on any modern system when
> IO/APIC is available, but the dependencies on the related checks cannot be
> resolved trivially and on short notice. This needs lots of analysis and
> rework.
> 
> The PIC detection has been added to avoid quirky checks and force selection
> of the dummy implementation all over the place, especially in VM guest
> scenarios. So it's not an option to revert the relevant commit as that
> would break a lot of other scenarios.
> 
> One solution would be to try to initialize the PIC on detection fail and
> retry the detection, but that puts the burden on everything which does not
> have a PIC.
> 
> Fortunately the ACPI/MADT table header has a flag field, which advertises
> in bit 0 that the system is PCAT compatible, which means it has a legacy
> 8259 PIC.
> 
> Evaluate that bit and if set avoid the detection routine and keep the real
> PIC installed, which then gets initialized (for nothing) and makes the rest
> of the code with all the dependencies work again.
> 
> Fixes: e179f6914152 ("x86, irq, pic: Probe for legacy PIC and set legacy_pic appropriately")
> Reported-by: David Lazar <dlazar@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Tested-by: David Lazar <dlazar@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218003

Thank you for this fix / workaround.

The patch looks good to me:

Reviewed-by: Hans de Goede <hdegoede@xxxxxxxxxx>

Regards,

Hans




> ---
> ---
>  arch/x86/include/asm/i8259.h |    2 ++
>  arch/x86/kernel/acpi/boot.c  |    3 +++
>  arch/x86/kernel/i8259.c      |   38 ++++++++++++++++++++++++++++++--------
>  3 files changed, 35 insertions(+), 8 deletions(-)
> 
> --- a/arch/x86/include/asm/i8259.h
> +++ b/arch/x86/include/asm/i8259.h
> @@ -69,6 +69,8 @@ struct legacy_pic {
>  	void (*make_irq)(unsigned int irq);
>  };
>  
> +void legacy_pic_pcat_compat(void);
> +
>  extern struct legacy_pic *legacy_pic;
>  extern struct legacy_pic null_legacy_pic;
>  
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -148,6 +148,9 @@ static int __init acpi_parse_madt(struct
>  		pr_debug("Local APIC address 0x%08x\n", madt->address);
>  	}
>  
> +	if (madt->flags & ACPI_MADT_PCAT_COMPAT)
> +		legacy_pic_pcat_compat();
> +
>  	/* ACPI 6.3 and newer support the online capable bit. */
>  	if (acpi_gbl_FADT.header.revision > 6 ||
>  	    (acpi_gbl_FADT.header.revision == 6 &&
> --- a/arch/x86/kernel/i8259.c
> +++ b/arch/x86/kernel/i8259.c
> @@ -32,6 +32,7 @@
>   */
>  static void init_8259A(int auto_eoi);
>  
> +static bool pcat_compat __ro_after_init;
>  static int i8259A_auto_eoi;
>  DEFINE_RAW_SPINLOCK(i8259A_lock);
>  
> @@ -299,15 +300,32 @@ static void unmask_8259A(void)
>  
>  static int probe_8259A(void)
>  {
> +	unsigned char new_val, probe_val = ~(1 << PIC_CASCADE_IR);
>  	unsigned long flags;
> -	unsigned char probe_val = ~(1 << PIC_CASCADE_IR);
> -	unsigned char new_val;
> +
> +	/*
> +	 * If MADT has the PCAT_COMPAT flag set, then do not bother probing
> +	 * for the PIC. Some BIOSes leave the PIC uninitialized and probing
> +	 * fails.
> +	 *
> +	 * Right now this causes problems as quite some code depends on
> +	 * nr_legacy_irqs() > 0 or has_legacy_pic() == true. This is silly
> +	 * when the system has an IO/APIC because then PIC is not required
> +	 * at all, except for really old machines where the timer interrupt
> +	 * must be routed through the PIC. So just pretend that the PIC is
> +	 * there and let legacy_pic->init() initialize it for nothing.
> +	 *
> +	 * Alternatively this could just try to initialize the PIC and
> +	 * repeat the probe, but for cases where there is no PIC that's
> +	 * just pointless.
> +	 */
> +	if (pcat_compat)
> +		return nr_legacy_irqs();
> +
>  	/*
> -	 * Check to see if we have a PIC.
> -	 * Mask all except the cascade and read
> -	 * back the value we just wrote. If we don't
> -	 * have a PIC, we will read 0xff as opposed to the
> -	 * value we wrote.
> +	 * Check to see if we have a PIC.  Mask all except the cascade and
> +	 * read back the value we just wrote. If we don't have a PIC, we
> +	 * will read 0xff as opposed to the value we wrote.
>  	 */
>  	raw_spin_lock_irqsave(&i8259A_lock, flags);
>  
> @@ -429,5 +447,9 @@ static int __init i8259A_init_ops(void)
>  
>  	return 0;
>  }
> -
>  device_initcall(i8259A_init_ops);
> +
> +void __init legacy_pic_pcat_compat(void)
> +{
> +	pcat_compat = true;
> +}
>