[RFC/PATCH 6/17][Kdump] NMI handler segment selector, stackpointer fix

[Vivek Goyal]

> --- Begin Message ---
> From: "Vivek Goyal" <vgoyal@xxxxxxxxxx>
>
> o CPU does not save ss and esp on stack if execution was already in kernel 
>   mode at the time of NMI occurrence. This leads to saving of erractic values
>   for ss and esp. This patch fixes the issue.  
>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> ---
>
>  linux-2.6.12-rc1-mm3-1M-root/arch/i386/kernel/crash.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+)
>
> diff -puN arch/i386/kernel/crash.c~x86-nmi-handler-ss-esp-fix arch/i386/kernel/crash.c
> --- linux-2.6.12-rc1-mm3-1M/arch/i386/kernel/crash.c~x86-nmi-handler-ss-esp-fix	2005-03-27 18:50:50.000000000 +0530
> +++ linux-2.6.12-rc1-mm3-1M-root/arch/i386/kernel/crash.c	2005-03-27 18:56:16.000000000 +0530
> @@ -112,7 +112,20 @@ static atomic_t waiting_for_crash_ipi;
>  
>  static int crash_nmi_callback(struct pt_regs *regs, int cpu)
>  {
> +	struct pt_regs fixed_regs;
>  	local_irq_disable();
> +
> +	/* CPU does not save ss and esp on stack if execution is already
> +	 * running in kernel mode at the time of NMI occurrence. This code
> +	 * fixes it.
> +	 */
> +	if (!user_mode(regs)) {
> +		memcpy(&fixed_regs, regs, sizeof(*regs));
> +		fixed_regs.esp = (unsigned long)&(regs->esp);
> +		__asm__ __volatile__("xorl %eax, %eax;");
> +		__asm__ __volatile__ ("movw %%ss, %%ax;" :"=a"(fixed_regs.xss));
> +		regs = &fixed_regs;
> +	}
>  	crash_save_this_cpu(regs, cpu);
>  	disable_local_APIC();
>  	atomic_dec(&waiting_for_crash_ipi);
> _
>
> --- End Message ---