On 5/10/25 7:05 AM, Paul Mackerras wrote:
> 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>
>
Thanks for the debug.
> 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?
I guess if we plan to go with adding r2 to clobber, it has to be gcc
specific, since my FC41 (gcc 14.1) complains of PIC registers usage
in the clobber list.
inlined from ‘__do_IRQ’ at arch/powerpc/kernel/irq.c:297:3:
arch/powerpc/kernel/irq.c:262:9: error: PIC register clobbered by ‘r2’ in ‘asm’
262 | asm volatile (
| ^~~
Can you try with this patch, I am testing this in my setup.
Maddy
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index a0e8b998c9b5..2ce7a4f2b2fb 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -276,7 +276,11 @@ static __always_inline void call_do_irq(struct pt_regs
*regs, void *sp)
[callee] "i" (__do_irq)
: // Clobbers
"lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
+#if __GNUC__ >= 15
+ "cr7", "r0", "r2", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
+#else
"cr7", "r0", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
+#endif
"r11", "r12"
);
}
>
> Thanks,
> Paul.
>
