Re: [PATCH v8 01/13] x86/apic: Construct a selector for the interrupt delivery mode

[Dou Liyang]

Hi Baoquan

I am wordy one ah:
our target is checking if BIOS supports APIC, no matter what
type(separated/integrated) it is. if not, go to PIC mode.

Letâs discuss the original logic and the smp_found_config,
then take about your code.

The existing logic is:

	if (!boot_cpu_has(X86_FEATURE_APIC) && !smp_found_config) ...(1)
		return -1;

	if (!boot_cpu_has(X86_FEATURE_APIC) &&
	                APIC_INTEGRATED(boot_cpu_apic_version)) { ...(2)
		pr_err(....);

why smp_found_config has to be checked in (1)?

Because, In case of discrete (pretty old) apics we may not set
X86_FEATURE_APIC bit in cpuid, with 82489DX we can't rely on apic
feature bit retrieved via cpuid(boot_cpu_has(X86_FEATURE_APIC)).[1]
So we assume that if SMP configuration is found from MP table
(smp_found_config = 1) in above case, there maybe a separated
chip in our pc.

After passing the check of (1), we in (2), check whether local APIC
is detected or not, If we have a BIOS bug.

[1] Commit 8312136fa8b0("x86, apic: Fix missed handling of discrete apics")

At 09/06/2017 06:17 PM, Baoquan He wrote:
> Hi Dou,
>
> On 08/28/17 at 11:20am, Dou Liyang wrote:
> > +static int __init apic_intr_mode_select(void)
> > +{
> > +	/* Check kernel option */
> > +	if (disable_apic) {
> > +		pr_info("APIC disabled via kernel command line\n");
> > +		return APIC_PIC;
> > +	}
> > +
>
> I am not very familiar with cpu registers, not sure if we can adjust
> below code flow as:
>
> 	/* If APIC is integrated, check local APIC only */
> 	if (lapic_is_integrated() && !boot_cpu_has(X86_FEATURE_APIC)) {
> 		disable_apic = 1;
> 		pr_info("APIC disabled by BIOS\n");
> 		return APIC_PIC;
> 	}
>
> 	/* If APIC is on a separate chip, check if smp_found_config is found*/
> 	if (!lapic_is_integrated() && !smp_found_config) {
> 		disable_apic = 1;
> 		return APIC_PIC;
> 	}

Yes, Awesome, we first consider it from APIC register space, then
the BOIS and software configration. let me do more investigation.

I rewrite it based on you, any comments will welcome.

	/* If APIC is not integrated, check if SMP configuration is
	 * found from MP table. If not too, no 82489DX. switch to
	 * PIC mode
	 *
	 * Else APIC is integrated, check if the BIOS allows local APIC
	 *
	 */
	if (!lapic_is_integrated()) {
		if (!smp_found_config) {
			disable_apic = 1;
			return APIC_PIC;
		}
	} else if(!boot_cpu_has(X86_FEATURE_APIC)) {
			disable_apic = 1;
			pr_info("APIC disabled by BIOS\n");
			return APIC_PIC;
		}
	}

BTW, As the macro APIC_INTEGRATED(x) has already wrapped by
CONFIG_X86_32, I will cleanup the lapic_is_integrated() for readablity
like that:

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 7834f73..63b3ae9 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -211,11 +211,7 @@ static inline int lapic_get_version(void)
  */
 static inline int lapic_is_integrated(void)
 {
-#ifdef CONFIG_X86_64
-       return 1;
-#else
        return APIC_INTEGRATED(lapic_get_version());
-#endif
 }


Do you think so. ;-)


Thanks,
	dou.


> 	~~~~ Now, I haven't think of why smp_found_config has to be
>              checked here.
>
> In this way, we don't need the CONFIG_X86_64 checking since it's
> contained in lapic_is_integrated() already. And the checking is obvious
> for understanding. Just not very sure if the checking is adequate.
>
> Just my personal opinion.
>
> > +	/* Check BIOS */
> > +#ifdef CONFIG_X86_64
> > +	/* On 64-bit, the APIC must be integrated, Check local APIC only */
> > +	if (!boot_cpu_has(X86_FEATURE_APIC)) {
> > +		disable_apic = 1;
> > +		pr_info("APIC disabled by BIOS\n");
> > +		return APIC_PIC;
> > +	}
> > +#else
> > +	/*
> > +	 * On 32-bit, check whether there is a separate chip or integrated
> > +	 * APIC
> > +	 */
> > +
> > +	/* Has a separate chip ? */
> > +	if (!boot_cpu_has(X86_FEATURE_APIC) && !smp_found_config) {
> > +		disable_apic = 1;
> > +
> > +		return APIC_PIC;
> > +	}
> > +
> > +	/* Has a local APIC ? */
> > +	if (!boot_cpu_has(X86_FEATURE_APIC) &&
> > +		APIC_INTEGRATED(boot_cpu_apic_version)) {
> > +		disable_apic = 1;
> > +		pr_err(FW_BUG "Local APIC %d not detected, force emulation\n",
> > +				       boot_cpu_physical_apicid);
> > +
> > +		return APIC_PIC;
> > +	}
> > +#endif
> > +
> > +	/* Check MP table or ACPI MADT configuration */
> > +	if (!smp_found_config) {
> > +		disable_ioapic_support();
> > +
> > +		if (!acpi_lapic)
> > +			pr_info("APIC: ACPI MADT or MP tables are not detected\n");
> > +
> > +		return APIC_VIRTUAL_WIRE;
> > +	}
> > +
> > +	return APIC_SYMMETRIC_IO;
> > +}
> > +
> >  /*
> >   * An initial setup of the virtual wire mode.
> >   */
> > --
> > 2.5.5