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

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Message-ID: <Zx-E_8SFV74s6xN8@J2N7QTR9R3>
Date: Mon, 28 Oct 2024 12:35:11 +0000
From: Mark Rutland <mark.rutland@....com>
To: Anshuman Khandual <anshuman.khandual@....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 Wed, Oct 23, 2024 at 11:42:37AM +0530, Anshuman Khandual wrote:
> 
> 
> 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 ?

Yes, that's why I said we need it regardless; it's an existing
requirement that wasn't documented.

> 
> > 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 ?

Yes please.

Mark.