[RFC PATCH] powerpc/xmon: Use OPAL_DEBUG to debug srest in OPAL

[Vasant Hegde]

On 03/27/2018 12:58 PM, Nicholas Piggin wrote:
> On Tue, 27 Mar 2018 12:42:32 +0530
> Vasant Hegde <hegdevasant at linux.vnet.ibm.com> wrote:
> 
>> On 03/26/2018 08:39 PM, Nicholas Piggin wrote:
>>> xmon can be entered via sreset NMI (from a management sreset, or an
>>> NMI IPI), which can interrupt OPAL. Add checks to xmon to see if pc
>>> or sp are within OPAL memory, and if so, then use OPAL_DEBUG to
>>> print the opal stack and return the Linux stack, which can then be
>>> dumped by xmon
>>
>> Nick,
>>
>>
>> OPAL uses FSP/cronus interface for many of debug interface (like OPAL assert,
>> getting opal console, triggering FSP R/R etc). May be in future we may add new
>> debug capability.
> 
> It would be good to ensure an API could accommodate them, or at least
> not get in the way.

Agree.

> 
>> Once secureboot is enabled none of these interface work and we have limited debug
>> capability.
>>
>> Here you are using very generic API name (OPAL_DEBUG). May be we should have generic
>> interface (exported via debugfs?) here rather than SRESET specific one.
> 
> OPAL_DEBUG here actually uses the sub-function OPAL_DEBUG_DUMP_STACK (1),
> but I didn't bring that constant across from skiboot which I should have.

Nick,

May be we should define sub-function usage.  Also current API limits number of 
arguments
and its type. may be we should have argument 2 as "void *" ?
something like :
   s64 opal_debug(u32 debug_type, void *data, u64 dsize);

That way individual sub-function can parse/use based on its need.

> 
> But I don't think this is SRESET specific. If Linux has any way to get
> an r1 for a CPU in OPAL, then it can use this function. If it does not,
> then it simply can't use it.
> 
> I haven't really followed what's happening with secure boot, but presumably
> we can still get NMIs (at least machine check, even if all system reset
> sources are suppressed).

AFAIK secureboot won't block us here. It mostly blocks external entity (like 
FSP/cronus) from
accessing host memory. (like they can't directly read, write to host memory, 
SCOM operations
are restricted etc).

-Vasant