Re: [PATCH] s390/setup: fix early warning messages

[Martin Schwidefsky]

On Mon, 18 Feb 2019 18:01:46 +0100
Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote:

> On Mon, 18 Feb 2019 07:46:40 -0800
> Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>    
> > 
> > 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:
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