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

[Dou Liyang]

Hi Baoquan,

At 09/07/2017 01:22 PM, Baoquan He wrote:
> 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.

OK, I will

> > 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 thought again and again, I would not change this check logic.

Because actually, we have three possibilities:

  1. ACPI on chip
  2. 82489DX
  3. no APIC

lapic_is_integrated() is used to check the APIC's type which is
APIC on chip or 82489DX. It has a prerequisite, we should avoid
the third possibility(no APIC) first, which is decided by
boot_cpu_has(X86_FEATURE_APIC) and smp_found_config. So, the original
logic:

if (!boot_cpu_has(X86_FEATURE_APIC) && !smp_found_config)

...is not just for 82489DX, but also for no APIC.

It looks more correct and understandable than us.

I am sorry my comments were wrong, and misled us. I will modify it
in my next version.

BTW, How about your test result, is this series OK?

Thanks,
	dou.