powerpc/vmx: avoid KASAN instrumentation in enter_vmx_ops() for kexec

powerpc/vmx: avoid KASAN instrumentation in enter_vmx_ops() for kexec

The kexec sequence invokes enter_vmx_ops() via copy_page() with the MMU
disabled. In this context, code must not rely on normal virtual address
translations or trigger page faults.

With KASAN enabled, functions get instrumented and may access shadow
memory using regular address translation. When executed with the MMU
off, this can lead to page faults (bad_page_fault) from which the
kernel cannot recover in the kexec path, resulting in a hang.

The kexec path sets preempt_count to HARDIRQ_OFFSET before entering
the MMU-off copy sequence.

current_thread_info()->preempt_count = HARDIRQ_OFFSET
  kexec_sequence(..., copy_with_mmu_off = 1)
    -> kexec_copy_flush(image)
         copy_segments()
           -> copy_page(dest, addr)
	         bl enter_vmx_ops()
                   if (in_interrupt())
                     return 0
	         beq .Lnonvmx_copy

Since kexec sets preempt_count to HARDIRQ_OFFSET, in_interrupt()
evaluates to true and enter_vmx_ops() returns early.

As in_interrupt() (and preempt_count()) are always inlined, mark
enter_vmx_ops() with __no_sanitize_address to avoid KASAN
instrumentation and shadow memory access with MMU disabled, helping
kexec boot fine with KASAN enabled.

Reported-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260407124349.1698552-2-sourabhjain@linux.ibm.com