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

[Baoquan He]

On 09/07/17 at 12:19pm, Dou Liyang wrote:
> 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")

Hmm, sounds reasonable. Just a sentence to describe it could be better.

> 
> 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;
> 		}
> 	}

Yeah, it's fine to me. At least the logic looks more understandable.

> 
> 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:

Yes, looks good. There's duplicate judgement of X86_64 in
lapic_is_integrated.

> 
> 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
> > > 
> > > 
> > > 
> > 
> > 
> > 
> 
>