[v3 PATCH 4/5] powerpc/pseries: Dump and flush SLB contents on SLB MCE errors.

[Michael Ellerman]

"Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com> writes:
> On 06/12/2018 07:17 PM, Michael Ellerman wrote:
>> Mahesh J Salgaonkar <mahesh at linux.vnet.ibm.com> writes:
>>> diff --git a/arch/powerpc/platforms/pseries/ras.c b/arch/powerpc/platforms/pseries/ras.c
>>> index 2edc673be137..e56759d92356 100644
>>> --- a/arch/powerpc/platforms/pseries/ras.c
>>> +++ b/arch/powerpc/platforms/pseries/ras.c
>>> @@ -422,6 +422,31 @@ int pSeries_system_reset_exception(struct pt_regs *regs)
>>>   	return 0; /* need to perform reset */
>>>   }
>>>   
>>> +static int mce_handle_error(struct rtas_error_log *errp)
>>> +{
>>> +	struct pseries_errorlog *pseries_log;
>>> +	struct pseries_mc_errorlog *mce_log;
>>> +	int disposition = rtas_error_disposition(errp);
>>> +	uint8_t error_type;
>>> +
>>> +	pseries_log = get_pseries_errorlog(errp, PSERIES_ELOG_SECT_ID_MCE);
>>> +	if (pseries_log == NULL)
>>> +		goto out;
>>> +
>>> +	mce_log = (struct pseries_mc_errorlog *)pseries_log->data;
>>> +	error_type = rtas_mc_error_type(mce_log);
>>> +
>>> +	if ((disposition == RTAS_DISP_NOT_RECOVERED) &&
>>> +			(error_type == PSERIES_MC_ERROR_TYPE_SLB)) {
>>> +		slb_dump_contents();
>>> +		slb_flush_and_rebolt();
>> 
>> Aren't we back in virtual mode here?
>> 
>> Don't we need to do the flush in real mode before turning the MMU back
>> on. Otherwise we'll just take another multi-hit?
>
> slb_flush_and_rebolt does slbia, which keeps slb index 0. So kernel code 
> should not get another slb miss. We also make sure we don't touch stack 
> in slb_flush_and_rebolt(). So we flush everything and put vmalloc and 
> stack back. That should be ok with MMU on?

I don't think so.

Imagine we take a multi-hit accessing the paca. The machine check is
delivered in real mode, so we can run and access the paca by it's real
address. But as soon as we turn the MMU back on, we'll take another
multi-hit when we access the paca.

If I'm reading the code right we are turning the MMU back on essentially
straight away when we rfid to machine_check_common().

cheers