linux-kernel - Re: [PATCH 2/3] arm64/boot: Enable EL2 requirements for FEAT

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Message-ID: <72700154-cbf4-4a0a-b6e2-6f0709dec0ce@arm.com>
Date: Wed, 23 Oct 2024 11:42:37 +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,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/3] arm64/boot: Enable EL2 requirements for
 FEAT_Debugv8p9



On 10/22/24 21:40, Mark Rutland wrote:
> On Tue, Oct 01, 2024 at 10:06:01AM +0530, Anshuman Khandual wrote:
>> Fine grained trap control for MDSELR_EL1 register needs to be configured in
>> HDFGRTR2_EL2, and HDFGWTR2_EL2 registers when kernel enters at EL1, but EL2
>> is also present. This adds a new helper __init_el2_fgt2() initializing this
>> new FEAT_FGT2 based fine grained registers.
>>
>> MDCR_EL2.EBWE needs to be enabled for additional (beyond 16) breakpoint and
>> watchpoint exceptions when kernel enters at EL1, but EL2 is also present.
>> This updates __init_el2_debug() as required for FEAT_Debugv8p9.
>>
>> While here, also update booting.rst with MDCR_EL3 and SCR_EL3 requirements.
> 
> [...]
> 
>> +  For CPUs with FEAT_Debugv8p9 extension present:
>> +
>> +  - If the kernel is entered at EL1 and EL2 is present:
>> +
>> +    - HDFGRTR2_EL2.nMDSELR_EL1 (bit 5) must be initialized to 0b1
>> +    - HDFGWTR2_EL2.nMDSELR_EL1 (bit 5) must be initialized to 0b1
>> +    - MDCR_EL2.EBWE (bit 43) must be initialized to 0b1
>> +
>> +  - If EL3 is present:
>> +
>> +    - MDCR_EL3.TDA (bit 9) must be initialized to 0b0
> 
> AFAICT we need TDA==0 this regardless of FEAT_Debugv8p9 (and e.g. we need

That's because MDCR_EL3.TDA=0, enables access to many other debug registers
beside FEAT_Debugv8p9, which are currently used and hence this MDCR_EL3.TDA
=0 requirement is a not a new one but rather a missing one instead ?

> MDCR_EL3.TPM==0 where FEAT_PMUv3 is implemented), so we should probably
> check if there's anything else we haven't yet documented in MDCR_EL3.

Will scan through MDCR_EL3 register and match it with existing documentation
i.e Documentation/arch/arm64/booting.rst. If there are some missing MDCR_EL3
fields which should be mentioned, will add them via a separate pre-requisite
patch ?

> 
> [...]
> 
>>  .Lskip_trace_\@:
>> +	mrs	x1, id_aa64dfr0_el1
>> +	ubfx	x1, x1, #ID_AA64DFR0_EL1_DebugVer_SHIFT, #4
>> +	cmp	x1, #ID_AA64DFR0_EL1_DebugVer_V8P9
>> +	b.lt	.Lskip_dbg_v8p9_\@
>> +
>> +	mov	x0, #MDCR_EL2_EBWE
>> +	orr	x2, x2, x0
> 
> That can be:
> 
> 	orr	x2, x2, #MDCR_EL2_EBWE

Right, will change.

> 
> Mark.