linux-kernel - Re: [PATCH 2/2] x86: Replace this_cpu_sp0 with current_top_of_stack and fix it on x86

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Date:	Thu, 26 Mar 2015 09:30:06 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...capital.net>, x86@...nel.org,
	linux-kernel@...r.kernel.org
CC:	Borislav Petkov <bp@...en8.de>, Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH 2/2] x86: Replace this_cpu_sp0 with current_top_of_stack
 and fix it on x86_32

On 03/06/2015 08:50 PM, Andy Lutomirski wrote:
> I broke 32-bit kernels.  The implementation of sp0 was correct as
> far as I can tell, but sp0 was much weirder on x86_32 than I
> realized.  It has the following issues:
>
>   - Init's sp0 is inconsistent with everything else's: non-init tasks
>     are offset by 8 bytes.  (I have no idea why, and the comment is unhelpful.)
>
>   - vm86 does crazy things to sp0.
>
> Fix it up by replacing this_cpu_sp0() with current_top_of_stack()
> and using a new percpu variable to track the top of the stack on
> x86_32.
>
> Fixes: 75182b1632a8 x86/asm/entry: Switch all C consumers of kernel_stack to this_cpu_sp0()
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---

...

> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index febc6aabc72e..759388c538cf 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -806,6 +806,8 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
>   #ifdef CONFIG_X86_32
>   	/* Stack for startup_32 can be just as for start_secondary onwards */
>   	irq_ctx_init(cpu);
> +	per_cpu(cpu_current_top_of_stack, cpu) =
> +		(unsigned long)task_stack_page(idle) + THREAD_SIZE;
>   #else
>   	clear_tsk_thread_flag(idle, TIF_FORK);
>   	initial_gs = per_cpu_offset(cpu);


Andy,

We need a similar change for Xen, otherwise 32-bit PV guests are not 
happy. Is the patch above final (and then should I submit a separate 
patch) or are you still working on it (and if so, please add the change 
below)?

-boris


diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 1c5e760..561d6f5 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -444,6 +444,8 @@ static int xen_cpu_up(unsigned int cpu, struct 
task_struct *idle)
         per_cpu(current_task, cpu) = idle;
  #ifdef CONFIG_X86_32
         irq_ctx_init(cpu);
+       per_cpu(cpu_current_top_of_stack, cpu) =
+                (unsigned long)task_stack_page(idle) + THREAD_SIZE;
  #else
         clear_tsk_thread_flag(idle, TIF_FORK);
  #endif

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