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 11:33:26 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	xen-devel <xen-devel@...ts.xen.org>,
	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86: Replace this_cpu_sp0 with current_top_of_stack
 and fix it on x86_32

On Mar 26, 2015 6:32 AM, "Boris Ostrovsky" <boris.ostrovsky@...cle.com> wrote:
>
> 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)?
>

My patch is final -- it's been in -tip for a while now.

It would be really nice if we could merge the bits of Xen and native
initialization that are identical rather than needing to duplicate all
this code.

--Andy
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