num_processors is 0 by default and only gets incremented when local APICs
are registered.

Make init_apic_mappings(), which tries to enable the local APIC in the case
that no SMP configuration was found set num_processors to 1.

This allows to remove yet another check for the local APIC and yet another
place which registers the boot CPUs local APIC ID.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/apic/apic.c |    9 ++++++---
 arch/x86/kernel/smpboot.c   |   18 ------------------
 2 files changed, 6 insertions(+), 21 deletions(-)

--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2130,9 +2130,12 @@ void __init init_apic_mappings(void)
 	if (x2apic_mode)
 		return;
 
-	if (!smp_found_config && !detect_init_APIC()) {
-		pr_info("APIC: disable apic facility\n");
-		apic_disable();
+	if (!smp_found_config) {
+		if (!detect_init_APIC()) {
+			pr_info("APIC: disable apic facility\n");
+			apic_disable();
+		}
+		num_processors = 1;
 	}
 }
 
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1397,24 +1397,6 @@ early_param("possible_cpus", _setup_poss
 {
 	int i, possible;
 
-	/* No processor was found in mptable or ACPI MADT */
-	if (!num_processors) {
-		if (boot_cpu_has(X86_FEATURE_APIC)) {
-			int apicid = boot_cpu_physical_apicid;
-			int cpu = read_apic_id();
-
-			pr_warn("Boot CPU (id %d) not listed by BIOS\n", cpu);
-
-			/* Make sure boot cpu is enumerated */
-			if (apic->cpu_present_to_apicid(0) == BAD_APICID &&
-			    apic->apic_id_valid(apicid))
-				generic_processor_info(apicid);
-		}
-
-		if (!num_processors)
-			num_processors = 1;
-	}
-
 	i = setup_max_cpus ?: 1;
 	if (setup_possible_cpus == -1) {
 		possible = num_processors;

On 24.07.23 15:34, Thomas Gleixner wrote:
> num_processors is 0 by default and only gets incremented when local APICs
> are registered.
> 
> Make init_apic_mappings(), which tries to enable the local APIC in the case
> that no SMP configuration was found set num_processors to 1.
> 
> This allows to remove yet another check for the local APIC and yet another
> place which registers the boot CPUs local APIC ID.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>   arch/x86/kernel/apic/apic.c |    9 ++++++---
>   arch/x86/kernel/smpboot.c   |   18 ------------------
>   2 files changed, 6 insertions(+), 21 deletions(-)
> 
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -2130,9 +2130,12 @@ void __init init_apic_mappings(void)
>   	if (x2apic_mode)
>   		return;
>   
> -	if (!smp_found_config && !detect_init_APIC()) {
> -		pr_info("APIC: disable apic facility\n");
> -		apic_disable();
> +	if (!smp_found_config) {
> +		if (!detect_init_APIC()) {
> +			pr_info("APIC: disable apic facility\n");
> +			apic_disable();
> +		}
> +		num_processors = 1;
>   	}
>   }
>   

This is introducing a regression for Xen PV guests: those have no ACPI
tables, so smp_found_config will be 0. OTOH num_processors has been set
already using hypervisor data, so setting num_processors to 1 here will
overwrite the previous setting.

Below diff on top is fixing the problem:

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 564d584c8b99..59c12b20c635 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2135,7 +2135,13 @@ void __init init_apic_mappings(void)
                         pr_info("APIC: disable apic facility\n");
                         apic_disable();
                 }
-               num_processors = 1;
+
+               /*
+                * Unprivileged Xen PV guests have smp_found_config = 0, but
+                * they have set num_processors already from hypervisor data.
+                */
+               if (!num_processors)
+                       num_processors = 1;
         }
  }


Juergen

On Mon, Jul 31 2023 at 12:17, Juergen Gross wrote:
> On 24.07.23 15:34, Thomas Gleixner wrote:
> This is introducing a regression for Xen PV guests: those have no ACPI
> tables, so smp_found_config will be 0. OTOH num_processors has been set
> already using hypervisor data, so setting num_processors to 1 here will
> overwrite the previous setting.
>
> Below diff on top is fixing the problem:

Fixing? You can't be serious about that.

Why can't XENPV pretend that it has a smp configuration detected,
i.e. setting smp_found_config as any other special get_smp_config()
implementation does?

XENPV is already a major pain to deal with. No need to expand the
related insanity all over the place.

Thanks,

        tglx

On Mon, Jul 31 2023 at 14:50, Thomas Gleixner wrote:
> Why can't XENPV pretend that it has a smp configuration detected,
> i.e. setting smp_found_config as any other special get_smp_config()
> implementation does?

The below should do the trick, no?


--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -182,7 +182,8 @@ static void __init _get_smp_config(unsig
 	if (subtract)
 		set_nr_cpu_ids(nr_cpu_ids - subtract);
 #endif
-
+	/* Pretend to be a proper enumerated system */
+	smp_found_config = 1;
 }
 
 static void __init xen_pv_smp_prepare_boot_cpu(void)

On 31.07.23 17:57, Thomas Gleixner wrote:
> On Mon, Jul 31 2023 at 14:50, Thomas Gleixner wrote:
>> Why can't XENPV pretend that it has a smp configuration detected,
>> i.e. setting smp_found_config as any other special get_smp_config()
>> implementation does?
> 
> The below should do the trick, no?

Something like that, yes.

I'm just hunting another regression in the series. With patch 23 of the
topology series applied the APs of a Xen PV guests won't be onlined. I
guess this is due to missing topology data initialization somewhere in
the Xen related code.

I'll check your suggestion after finding the reason for the regression.


Juergen

> 
> 
> --- a/arch/x86/xen/smp_pv.c
> +++ b/arch/x86/xen/smp_pv.c
> @@ -182,7 +182,8 @@ static void __init _get_smp_config(unsig
>   	if (subtract)
>   		set_nr_cpu_ids(nr_cpu_ids - subtract);
>   #endif
> -
> +	/* Pretend to be a proper enumerated system */
> +	smp_found_config = 1;
>   }
>   
>   static void __init xen_pv_smp_prepare_boot_cpu(void)

On 31.07.23 20:19, Juergen Gross wrote:
> On 31.07.23 17:57, Thomas Gleixner wrote:
>> On Mon, Jul 31 2023 at 14:50, Thomas Gleixner wrote:
>>> Why can't XENPV pretend that it has a smp configuration detected,
>>> i.e. setting smp_found_config as any other special get_smp_config()
>>> implementation does?
>>
>> The below should do the trick, no?
> 
> Something like that, yes.
> 
> I'm just hunting another regression in the series. With patch 23 of the
> topology series applied the APs of a Xen PV guests won't be onlined. I
> guess this is due to missing topology data initialization somewhere in
> the Xen related code.
> 
> I'll check your suggestion after finding the reason for the regression.
> 
>>
>>
>> --- a/arch/x86/xen/smp_pv.c
>> +++ b/arch/x86/xen/smp_pv.c
>> @@ -182,7 +182,8 @@ static void __init _get_smp_config(unsig
>>       if (subtract)
>>           set_nr_cpu_ids(nr_cpu_ids - subtract);
>>   #endif
>> -
>> +    /* Pretend to be a proper enumerated system */
>> +    smp_found_config = 1;
>>   }
>>   static void __init xen_pv_smp_prepare_boot_cpu(void)
> 

It is working fine.


Juergen