linux-kernel - Re: [PATCH 1/3] arm64/cpufeature: Add field details for ID_AA64DFR1

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Message-ID: <8efe902c-8b9f-494a-b9da-430d8ced32ef@arm.com>
Date: Mon, 28 Oct 2024 19:08:11 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, James Morse <james.morse@....com>,
 Suzuki K Poulose <suzuki.poulose@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Mark Brown <broonie@...nel.org>, kvmarm@...ts.linux.dev
Subject: Re: [PATCH 1/3] arm64/cpufeature: Add field details for
 ID_AA64DFR1_EL1 register

On 10/28/24 18:03, Mark Rutland wrote:
> On Wed, Oct 23, 2024 at 11:18:12AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 10/22/24 21:26, Mark Rutland wrote:
>>> On Tue, Oct 01, 2024 at 10:06:00AM +0530, Anshuman Khandual wrote:
>>>> This adds required field details for ID_AA64DFR1_EL1, and also drops dummy
>>>> ftr_raz[] array which is now redundant. These register fields will be used
>>>> to enable increased breakpoint and watchpoint registers via FEAT_Debugv8p9
>>>> later.
> 
>>>> +static const struct arm64_ftr_bits ftr_id_aa64dfr1[] = {
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_ABL_CMPs_SHIFT, 8, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_DPFZS_SHIFT, 4, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_EBEP_SHIFT, 4, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_ITE_SHIFT, 4, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_ABLE_SHIFT, 4, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_PMICNTR_SHIFT, 4, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_SPMU_SHIFT, 4, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_CTX_CMPs_SHIFT, 8, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_WRPs_SHIFT, 8, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_BRPs_SHIFT, 8, 0),
>>>> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR1_EL1_SYSPMUID_SHIFT, 8, 0),
>>>> +	ARM64_FTR_END,
>>>> +};
>>>> +
>>>
>>> Is there some general principle that has been applied here? e.g. is this
>>> STRICT unless we know of variation in practice?
>>
>> Yes, that's correct. STRICT unless there is a known variation in practice.
> 
> Please mention that somewhere, e.g. in the commit message.

Sure, will add that.

> 
>>> e.g. it seems a bit odd that ABLE cannot vary while the number of
>>> breakpoints can...
>> But all these (ABL_CMPs, CTX_CMPs, WRPs, BRPs) are marked as FTR_NONSTRICT.
>> Would not that allow ABL_CMPs to vary as well ?
> 
> I asked about ABLE, not ABL_CMPs.
> 
> ABL_CMPs is marked as FTR_NONSTRICT, but ABLE is marked as FTR_STRICT.

Ahh, that was my bad, completely missed.

> 
>> Although the existing break-point numbers are currently marked FTR_STRICT,
>> should they be changed first ?
>>
>> static const struct arm64_ftr_bits ftr_id_aa64dfr0[] = {
>> 	...................
>> 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR0_EL1_CTX_CMPs_SHIFT, 4, 0),
>>         ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR0_EL1_WRPs_SHIFT, 4, 0),
>>         ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64DFR0_EL1_BRPs_SHIFT, 4, 0),
>> 	...................
>> }
> 
> My point was that the above didn't seem to be logically consistent; I
> think you didn't handle ABLE as you should have.

Agreed, will change ABLE as FTR_NONSTRICT instead.

But what about the ID_AA64DFR0_EL1_WRPs_SHIFT and ID_AA64DFR0_EL1_BRPs_SHIFT
which could have variations in different cpus on the same system ? So should
those be fixed as FTR_NONSTRICT first ?

I have posted V2 for this series earlier today, hence will accommodate all the
new comments here in V3 going forward.

https://lore.kernel.org/all/20241028053426.2486633-1-anshuman.khandual@arm.com/