Re: [PATCH 14/15] mips: panic if vector register partitioning is implemented

[David Daney <ddaney.cavm@gmail.com>]

On 01/27/2014 11:39 AM, Paul Burton wrote:
> On Mon, Jan 27, 2014 at 10:38:45AM -0800, David Daney wrote:
>> ....
>> On 01/27/2014 07:23 AM, Paul Burton wrote:
>>> No current systems implementing MSA include support for vector register
>>> partitioning which makes it somewhat difficult to implement support for
>>> it in the kernel. Thus for the moment the kernel includes no such
>>> support. However if the kernel were to be run on a system which
>>> implemented register partitioning then it would not function correctly,
>>> mishandling MSA disabled exceptions. Calling panic when run on a system
>>> with vector register partitioning implemented ensures that we're not
>>> caught out by this later but instead reminded to implement support once
>>> such a system is available.
>>>
>>> Signed-off-by: Paul Burton <paul.burton@imgtec.com>
>>> ---
>>>   arch/mips/kernel/cpu-probe.c | 6 +++++-
>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
>>> index 852e085..003ba3c 100644
>>> --- a/arch/mips/kernel/cpu-probe.c
>>> +++ b/arch/mips/kernel/cpu-probe.c
>>> @@ -1193,9 +1193,13 @@ void cpu_probe(void)
>>>   	else
>>>   		c->srsets = 1;
>>>
>>> -	if (cpu_has_msa)
>>> +	if (cpu_has_msa) {
>>>   		c->msa_id = cpu_get_msa_id();
>>>
>>> +		if (c->msa_id & MSA_IR_WRPF)
>>> +			panic("Vector register partitioning unimplemented!");
>>
>> You should probably use a WARN_ON() instead.  There is no reason to crash
>> the kernel for this condition is there?
>>
>
> Well mapping vector registers reuses the MSA disabled exception, so if
> the kernel were to continue with my current code & userland were to
> execute an MSA instruction I believe it would appear to hang. [...]

The CPU probing things are called so early that any panic() or BUG() 
here will result in absolutely no console output as this code is called 
before any console drivers are enabled.

So the choice is really:

panic(): No output on console and system is frozen/locked-up.

WARN(): Nice stack trace on console, theoretical lockup once userspace 
code starts executing.

You can probably guess which I think is the better option.

>
> Thanks,
>      Paul
>
>>> +	}
>>> +
>>>   	cpu_probe_vmbits(c);
>>>
>>>   #ifdef CONFIG_64BIT
>>>
>>
>>
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>
>
>