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

Let‘s 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
>>
>>
>>
>
>
>


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