x86_64 write apic id fix

o Apic id is in most significant 8 bits of APIC_ID register. Current code is trying to write apic id to least significant 8 bits. This patch fixes it.

o This fix enables booting uni kdump capture kernel on a cpu with non-zero apic id.

---

diff -puN arch/x86_64/kernel/apic.c~kdump-up-kernel-non-boot-cpu-fix arch/x86_64/kernel/apic.c
@@ -1060,7 +1060,7 @@ int __init APIC_init_uniprocessor (void)
 	connect_bsp_APIC();
 
 	phys_cpu_present_map = physid_mask_of_physid(boot_cpu_id);
-	apic_write_around(APIC_ID, boot_cpu_id);
+	apic_write_around(APIC_ID, SET_APIC_ID(boot_cpu_id));
 
 	setup_local_APIC();
 

diff -puN include/asm-x86_64/apicdef.h~kdump-up-kernel-non-boot-cpu-fix include/asm-x86_64/apicdef.h
@@ -13,6 +13,7 @@
 #define		APIC_ID		0x20
 #define			APIC_ID_MASK		(0xFFu<<24)
 #define			GET_APIC_ID(x)		(((x)>>24)&0xFFu)
+#define			SET_APIC_ID(x)		(((x)<<24))
 #define		APIC_LVR	0x30
 #define			APIC_LVR_MASK		0xFF00FF
 #define			GET_APIC_VERSION(x)	((x)&0xFFu)

_

Thanks merged.
-Andi

What difference does it make in the first place? In APIC_init_uniprocessor() you write back the apic id read before from processor, not from mp table (because you wouldn't be in APIC_init_uniprocessor() otherwise).

--Mika

In this case APIC_init_uniprocessor() is being called from smp_init() as I have compiled this as a uni kernel. And smp_found_config is 1. So this can not be guaranteed that boot_cpu_id has been read from APIC.

But at the same time I see two functions (detect_init_APIC() and init_apic_mappings()) which overwrite the boot_cpu_id unconditionally. Former sets it to zero by default and later always reads it from APIC. This basically means that we just don't trust the MP tables in all the cases.

For kdump case, this becomes further tricky, as we boot into a second kernel without going through the BIOS and we might be booting on a cpu with non-zero apic id. But ACPI and MP tables will continue to report boot cpu id as 0 and that's not correct.

To come back to your question, I have just fixed one problem and that is if you are writing boot_cpu_id to APIC_ID register, write it in a proper manner. Otherwise what happens in this case is that boot_cpu_id is 1 and if I use apic_write_around(apic, boot_cpu_id) then it writes 0 to APIC_ID register instead of 1 and definitely that's not the requirement.

Thanks
Vivek