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! -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.
