s390x/tcg: Don't ignore content in r0 when not specified via "b" or "x"

s390x/tcg: Don't ignore content in r0 when not specified via "b" or "x"

Using get_address() with register identifiers comming from an "r" field
is wrong: if the "r" field designates "r0", we don't read the content
and instead assume 0 - which should only be applied when the register
was specified via "b" or "x".

PoP 5-11 "Operand-Address Generation":
  "A zero in any of the B1, B2, X2, B3, or B4 fields indicates the absence
   of the corresponding address component. For the absent component, a zero
   is used in forming the intermediate sum, regardless of the contents of
   general register 0. A displacement of zero has no special significance."

This BUG became visible for CSPG as generated by LLVM-12 in the upstream
Linux kernel (v5.11-rc2), used while creating the linear mapping in
vmem_map_init(): Trying to store to address 0 results in a Low Address
Protection exception.

Debugging this was more complicated than it could have been: The program
interrupt handler in the kernel will try to crash the kernel: doing so, it
will enable DAT. As the linear mapping is not created yet (asce=0), we run
into an addressing exception while tring to walk non-existant DAT tables,
resulting in a program exception loop.

This allows for booting upstream Linux kernels compiled by clang-12. Most
of these cases seem to be broken forever.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210111163845.18148-4-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>