Re: [PATCH V3 02/16] arm64/cpufeature: Drop TraceFilt feature exposure from ID_DFR0 register

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 05/05/2020 01:54 AM, Will Deacon wrote:
> On Sat, May 02, 2020 at 07:03:51PM +0530, Anshuman Khandual wrote:
>> ID_DFR0 based TraceFilt feature should not be exposed to guests. Hence lets
>> drop it.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Marc Zyngier <maz@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: James Morse <james.morse@arm.com>
>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>>
>> Suggested-by: Mark Rutland <mark.rutland@arm.com>
>> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>>  arch/arm64/kernel/cpufeature.c | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 6d032fbe416f..51386dade423 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -435,7 +435,6 @@ static const struct arm64_ftr_bits ftr_id_pfr1[] = {
>>  };
>>  
>>  static const struct arm64_ftr_bits ftr_id_dfr0[] = {
>> -	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 28, 4, 0),
> 
> Hmm, this still confuses me. Is this not now FTR_NONSTRICT? Why is that ok?

Mark had mentioned about it earlier (https://patchwork.kernel.org/patch/11287805/)
Did I misinterpret the first part ? Could not figure "capping the emulated debug
features" part. Probably, Mark could give some more details.

From the earlier discussion:

* ID_DFR0 fields need more thought; we should limit what we expose here.
  I don't think it's valid for us to expose TraceFilt, and I suspect we
  need to add capping for debug features we currently emulate.