Re: [PATCH] fix broken x86 SMP boot sequence

[Zachary Amsden]

James Bottomley wrote:

> This patch:
>  

I'm not adverse to making the gdt descriptors part of the per-cpu data.  
But I think your patch is missing some necessary changes to 
include/asm-i386/desc.h - what do you plan to do with the following 
definition there?

extern struct Xgt_desc_struct idt_descr, cpu_gdt_descr[NR_CPUS];


Also, it seems likely you may have broken APM and/or PnP BIOS.

> diff --git a/arch/i386/kernel/cpu/common.c b/arch/i386/kernel/cpu/common.c
> index cbc3206..d561a56 100644
> --- a/arch/i386/kernel/cpu/common.c
> +++ b/arch/i386/kernel/cpu/common.c
> @@ -4,6 +4,7 @@
> #include <linux/smp.h>
> #include <linux/module.h>
> #include <linux/percpu.h>
> +#include <linux/bootmem.h>
> #include <asm/semaphore.h>
> #include <asm/processor.h>
> #include <asm/i387.h>
> @@ -18,6 +19,9 @@
>
> #include "cpu.h"
>
> +DEFINE_PER_CPU(struct Xgt_desc_struct, cpu_gdt_descr);
> +EXPORT_PER_CPU_SYMBOL(cpu_gdt_descr);
> +
> DEFINE_PER_CPU(unsigned char, cpu_16bit_stack[CPU_16BIT_STACK_SIZE]);
> EXPORT_PER_CPU_SYMBOL(cpu_16bit_stack);
>
> @@ -559,8 +563,9 @@ void __devinit cpu_init(void)
> 	int cpu = smp_processor_id();
> 	struct tss_struct * t = &per_cpu(init_tss, cpu);
> 	struct thread_struct *thread = &current->thread;
> -	struct desc_struct *gdt = get_cpu_gdt_table(cpu);
> +	struct desc_struct *gdt;
> 	__u32 stk16_off = (__u32)&per_cpu(cpu_16bit_stack, cpu);
> +	struct Xgt_desc_struct *cpu_gdt_descr = &per_cpu(cpu_gdt_descr, cpu);
>
> 	if (cpu_test_and_set(cpu, cpu_initialized)) {
> 		printk(KERN_WARNING "CPU#%d already initialized!\n", cpu);
> @@ -577,6 +582,22 @@ void __devinit cpu_init(void)
> 		set_in_cr4(X86_CR4_TSD);
> 	}
>
> +	/* This is a horrible hack to allocate the GDT.  The problem
> +	 * is that cpu_init() is called really early for the boot CPU
> +	 * (and hence needs bootmem) but much later for the secondary
> +	 * CPUs, when bootmem will have gone away */
> +	if (NODE_DATA(0)->bdata->node_bootmem_map) {
> +		gdt = (struct desc_struct *)alloc_bootmem_pages(PAGE_SIZE);
> +		/* alloc_bootmem_pages panics on failure, so no check */
> +		memset(gdt, 0, PAGE_SIZE);
> +	} else {
> +		gdt = (struct desc_struct *)get_zeroed_page(GFP_KERNEL);
> +		if (unlikely(!gdt)) {
> +			printk(KERN_CRIT "CPU%d failed to allocate GDT\n", cpu);
> +			for (;;) local_irq_enable();
> +		}
> +	}
> +
> 	/*
> 	 * Initialize the per-CPU GDT with the boot GDT,
> 	 * and set up the GDT descriptor:
>  


Can't you get rid of this ugly hack _and_ optimize the code at the same 
time?  I don't understand the details of voyager bringup, but it seems 
clear that the BSP or node 0 is special in both sub-architectures.  If 
it is special anyway, the conditional logic can be merged, and better 
yet, the allocation for the first caller of cpu_init() can skip the 
allocation entirely - it can simply use the boot GDT, which is already 
page-aligned and ready to go.  This gets rid of the 
alloc_bootmem_pages() piece of the hack above.

Also, it seems you left the allocation of the GDT in do_boot_cpu().  How 
do you plan to make sure that GDT allocation is compatible with hotplug 
CPU startup?  I was worried about that, which is why I moved the GDT 
allocation for secondary processors (originally in 
arch/i386/kernel/cpu/common.c) to smpboot.c.

Zach
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