powerpc64/bpf: Implement JIT support for private stack

powerpc64/bpf: Implement JIT support for private stack

Provision the private stack as a per-CPU allocation during
bpf_int_jit_compile(). Align the stack to 16 bytes and place guard
regions at both ends to detect runtime stack overflow and underflow.

Round the private stack size up to the nearest 16-byte boundary.
Make each guard region 16 bytes to preserve the required overall
16-byte alignment. When private stack is set, skip bpf stack size
accounting in kernel stack.

There is no stack pointer in powerpc. Stack referencing during JIT
is done using frame pointer. Frame pointer calculation goes like:

BPF frame pointer = Priv stack allocation start address +
                    Overflow guard +
                    Actual stack size defined by verifier

Memory layout:

High Addr          +--------------------------------------------------+
                   |                                                  |
                   | 16 bytes Underflow guard (0xEB9F12345678eb9fULL) |
                   |                                                  |
         BPF FP -> +--------------------------------------------------+
                   |                                                  |
                   | Private stack - determined by verifier           |
                   | 16-bytes aligned                                 |
                   |                                                  |
                   +--------------------------------------------------+
                   |                                                  |
Lower Addr         | 16 byte Overflow guard (0xEB9F12345678eb9fULL)   |
                   |                                                  |
Priv stack alloc ->+--------------------------------------------------+
start

Update BPF_REG_FP to point to the calculated offset within the
allocated private stack buffer. Now, BPF stack usage reference
in the allocated private stack.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260401103215.104438-1-adubey@linux.ibm.com