Crash in __do_IRQ with gcc 15

[Paul Mackerras <paulus@ozlabs.org>]

Running Linux on Microwatt with a kernel compiled on an x86-64 system
running Fedora 42 (using the packaged cross-compiler, i.e. the
gcc-powerpcle64-linux-gnu package), I'm seeing a crash like this:

[    0.141591] smp: Bringing up secondary CPUs ...
[    0.167628] BUG: Unable to handle kernel data access on write at 0xc00a0000be8d6004
[    0.175409] Faulting instruction address: 0xc00000000000fcb4
cpu 0x0: Vector: 300 (Data Access) at [c0000000012f78d0]
    pc: c00000000000fcb4: __do_IRQ+0x64/0x84
    lr: c00000000000fccc: __do_IRQ+0x7c/0x84
    sp: c0000000012f7b70
   msr: 9000000000001033
   dar: c00a0000be8d6004
 dsisr: 42000000
  current = 0xc0000000012de000
  paca    = 0xc00000000135d000   irqmask: 0x03   irq_happened: 0x01
    pid   = 0, comm = swapper/0
Linux version 6.15.0-rc1-00001-g72b73737d483-dirty (paulus@thinks) (powerpc64le-linux-gnu-gcc (GCC) 15.0.1 20250329 (Red Hat Cross 15.0.1-0), GNU ld version 2.44-1.fc42) #5 SMP Thu May  8 22:20:34 AEST 2025
enter ? for help
[c0000000012f7b70] c00000000000fd50 do_IRQ+0x7c/0x90 (unreliable)
[c0000000012f7ba0] c000000000007db4 hardware_interrupt_common_virt+0x1c4/0x1d0
--- Exception: 500 (Hardware Interrupt) at c00000000001c2ec arch_local_irq_restore+0x60/0xc4
[c0000000012f7ea0] c000000000083c68 do_idle+0xd4/0xf4
[c0000000012f7ee0] c000000000083e08 cpu_startup_entry+0x34/0x38
[c0000000012f7f10] c00000000000cc7c kernel_init+0x0/0x144
[c0000000012f7f40] c000000001000ecc do_one_initcall+0x0/0x160
[c0000000012f7fe0] c00000000000ba6c start_here_common+0x1c/0x20
0:mon>

What's happening is that gcc 15 seems to be using r2 as an ordinary
register, and r2 has a live value in it at the point where __do_IRQ()
calls call_do_irq().  Since r2 is not in the clobber list for the
inline asm in call_do_irq(), it doesn't get saved and restored around
the call to __do_irq(), and when we come back to __do_IRQ(), it has
been modified.  Then when __do_IRQ() subsequently does a store using
r2, it blows up like the above.

Adding r2 to the clobber list in call_do_irq() fixes it.  Does this
seem like the right fix?  Does it need to be conditional on the gcc
version?  Or is there a better way to fix the problem?

Thanks,
Paul.