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Date:   Mon, 18 Feb 2019 11:22:40 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Cornelia Huck <cohuck@...hat.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] s390/setup: fix early warning messages

Hi Cornelia,

On 2/18/19 10:16 AM, Cornelia Huck wrote:
> On Mon, 18 Feb 2019 18:21:06 +0100
> Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> 
>> On Mon, 18 Feb 2019 18:01:46 +0100
>> Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
>>
>>> On Mon, 18 Feb 2019 07:46:40 -0800
>>> Guenter Roeck <linux@...ck-us.net> wrote:
>>>    
>>>> Hi,
>>>>
>>>> On Thu, Feb 14, 2019 at 03:40:56PM +0100, Martin Schwidefsky wrote:
>>>>> The setup_lowcore() function creates a new prefix page for the boot CPU.
>>>>> The PSW mask for the system_call, external interrupt, i/o interrupt and
>>>>> the program check handler have the DAT bit set in this new prefix page.
>>>>>
>>>>> At the time setup_lowcore is called the system still runs without virtual
>>>>> address translation, the paging_init() function creates the kernel page
>>>>> table and loads the CR13 with the kernel ASCE.
>>>>>
>>>>> Any code between setup_lowcore() and the end of paging_init() that has
>>>>> a BUG or WARN statement will create a program check that can not be
>>>>> handled correctly as there is no kernel page table yet.
>>>>>
>>>>> To allow early WARN statements initially setup the lowcore with DAT off
>>>>> and set the DAT bit only after paging_init() has completed.
>>>>>
>>>>> Cc: stable@...r.kernel.org
>>>>> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
>>>>
>>>> This patch causes s390 qemu emulations to crash with a kernel stack overflow.
>>>> Reverting the patch fixes the problem. Crash log and bisect results below.
>>>
>>> Urgs, yes. That is EDAT-1 again that makes it work with 1MB pages but breaks
>>> with 4K mapping where the prefix page is mapped to absolute zero.
>>>
>>> Just using S390_lowcore instead of lowcore_ptr[0] does not work either
>>> because low-address protection is already active. I'll think of something.
>>>
>>> Thanks for bug report!
>>   
>> This patch should fix the problem:
>> --
>>  From d4393e82c3ec9b2fe5dba4b0d1b6eef29f8d15c8 Mon Sep 17 00:00:00 2001
>> From: Martin Schwidefsky <schwidefsky@...ibm.com>
>> Date: Mon, 18 Feb 2019 18:10:08 +0100
>> Subject: [PATCH] s390/setup: fix boot crash for machine without EDAT-1
>>
>> The fix to make WARN work in the early boot code created a problem
>> on older machines without EDAT-1. The setup_lowcore_dat_on function
>> uses the pointer from lowcore_ptr[0] to set the DAT bit in the new
>> PSWs. That does not work if the kernel page table is set up with
>> 4K pages as the prefix address maps to absolute zero.
>>
>> To make this work the PSWs need to be changed with via address 0 in
>> form of the S390_lowcore definition.
>>
>> Cc: stable@...r.kernel.org
>> Fixes: 94f85ed3e2 ("s390/setup: fix early warning messages")
>> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
>> ---
>>   arch/s390/kernel/setup.c | 13 ++++++-------
>>   1 file changed, 6 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
>> index 65b22ef5141a..12934e8fbb91 100644
>> --- a/arch/s390/kernel/setup.c
>> +++ b/arch/s390/kernel/setup.c
>> @@ -451,13 +451,12 @@ static void __init setup_lowcore_dat_off(void)
>>   
>>   static void __init setup_lowcore_dat_on(void)
>>   {
>> -	struct lowcore *lc;
>> -
>> -	lc = lowcore_ptr[0];
>> -	lc->external_new_psw.mask |= PSW_MASK_DAT;
>> -	lc->svc_new_psw.mask |= PSW_MASK_DAT;
>> -	lc->program_new_psw.mask |= PSW_MASK_DAT;
>> -	lc->io_new_psw.mask |= PSW_MASK_DAT;
>> +	__ctl_clear_bit(0, 28);
>> +	S390_lowcore.external_new_psw.mask |= PSW_MASK_DAT;
>> +	S390_lowcore.svc_new_psw.mask |= PSW_MASK_DAT;
>> +	S390_lowcore.program_new_psw.mask |= PSW_MASK_DAT;
>> +	S390_lowcore.io_new_psw.mask |= PSW_MASK_DAT;
>> +	__ctl_set_bit(0, 28);
>>   }
>>   
>>   static struct resource code_resource = {
> 
> I could reproduce the crash under qemu/tcg and with this patch on top
> it is gone.
> 

What is your qemu command line ?

Thanks,
Guenter

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