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Subject: Re: [PATCH 1/2] powerpc: Introduce POWER10_DD1 feature
To: Michael Ellerman <mpe@ellerman.id.au>
References: <20201020054454.194343-1-ravi.bangoria@linux.ibm.com>
 <87ft66xzm7.fsf@mpe.ellerman.id.au>
From: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Message-ID: <911f16ea-352f-2764-d11b-474f9e8ebffa@linux.ibm.com>
Date: Mon, 26 Oct 2020 15:28:36 +0530
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Cc: christophe.leroy@c-s.fr, Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
 mikey@linux.ibm.com, jniethe5@gmail.com, npiggin@gmail.com,
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Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi Michael,

>> +static void __init fixup_cpu_features(void)
>> +{
>> +	unsigned long version = mfspr(SPRN_PVR);
>> +
>> +	if ((version & 0xffffffff) == 0x00800100)
>> +		cur_cpu_spec->cpu_features |= CPU_FTR_POWER10_DD1;
>> +}
>> +
>>   static int __init early_init_dt_scan_cpus(unsigned long node,
>>   					  const char *uname, int depth,
>>   					  void *data)
>> @@ -378,6 +386,7 @@ static int __init early_init_dt_scan_cpus(unsigned long node,
>>   
>>   		check_cpu_feature_properties(node);
>>   		check_cpu_pa_features(node);
>> +		fixup_cpu_features();
>>   	}
> 
> This is not the way we normally do CPU features.
> 
> In the past we have always added a raw entry in cputable.c, see eg. the
> Power9 DD 2.0, 2.1 entries.

True. But that won't work PowerVM guests because kernel overwrites "raw"
mode cpu_features with "architected" mode cpu_features.

> 
> Doing it here is not really safe, if you're running with an architected
> PVR (via cpu-version property), you can't set the DD1 feature, because
> you might be migrated to a future CPU that doesn't have the DD1 quirks.

Okay.. I suppose, that mean kernel need to check real PVR every time when it
encounters spurious exception. i.e. instead of using

   if (cpu_has_feature(CPU_FTR_POWER10_DD1) && ...)

it need to use

   if (mfspr(SPRN_PVR) == 0x800100 && ...)

in patch 2. Right?

Thanks,
Ravi