[v3 7/7] powerpc/64s: save r13 in MCE handler (simulator workaroud)

[Nicholas Piggin]

Santosh Sivaraj's on July 6, 2019 7:26 am:
> From: Reza Arbab <arbab at linux.ibm.com>
> 
> Testing my memcpy_mcsafe() work in progress with an injected UE, I get
> an error like this immediately after the function returns:
> 
> BUG: Unable to handle kernel data access at 0x7fff84dec8f8
> Faulting instruction address: 0xc0080000009c00b0
> Oops: Kernel access of bad area, sig: 11 [#1]
> LE PAGE_SIZE=64K MMU=Radix MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
> Modules linked in: mce(O+) vmx_crypto crc32c_vpmsum
> CPU: 0 PID: 1375 Comm: modprobe Tainted: G           O      5.1.0-rc6 #267
> NIP:  c0080000009c00b0 LR: c0080000009c00a8 CTR: c000000000095f90
> REGS: c0000000ee197790 TRAP: 0300   Tainted: G           O       (5.1.0-rc6)
> MSR:  900000000280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 88002826  XER: 00040000
> CFAR: c000000000095f8c DAR: 00007fff84dec8f8 DSISR: 40000000 IRQMASK: 0
> GPR00: 000000006c6c6568 c0000000ee197a20 c0080000009c8400 fffffffffffffff2
> GPR04: c0080000009c02e0 0000000000000006 0000000000000000 c000000003c834c8
> GPR08: 0080000000000000 776a6681b7fb5100 0000000000000000 c0080000009c01c8
> GPR12: c000000000095f90 00007fff84debc00 000000004d071440 0000000000000000
> GPR16: 0000000100000601 c0080000009e0000 c000000000c98dd8 c000000000c98d98
> GPR20: c000000003bba970 c0080000009c04d0 c0080000009c0618 c0000000001e5820
> GPR24: 0000000000000000 0000000000000100 0000000000000001 c000000003bba958
> GPR28: c0080000009c02e8 c0080000009c0318 c0080000009c02e0 0000000000000000
> NIP [c0080000009c00b0] cause_ue+0xa8/0xe8 [mce]
> LR [c0080000009c00a8] cause_ue+0xa0/0xe8 [mce]
> 
> After debugging we see that the first instruction at vector 200 is skipped by
> the simulator, due to which r13 is not saved. Adding a nop at 0x200 fixes the
> issue.
> 
> (This commit is needed for testing this series. This should not be taken
> into the tree)

Would be good if this was testable in simulator upstream, did you
report it? What does cause_ue do? exc_mce in mambo seems to do the
right thing AFAIKS.

Thanks,
Nick

> ---
>  arch/powerpc/kernel/exceptions-64s.S | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> index 73ba246ca11d..8e43abb2a744 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -255,6 +255,7 @@ EXC_REAL_BEGIN(machine_check, 0x200, 0x100)
>  	 * some code path might still want to branch into the original
>  	 * vector
>  	 */
> +	nop
>  	SET_SCRATCH0(r13)		/* save r13 */
>  	EXCEPTION_PROLOG_0(PACA_EXMC)
>  BEGIN_FTR_SECTION
> -- 
> 2.20.1
> 
>