Re: [PATCH 2/3] arm64/boot: Enable EL2 requirements for FEAT_Debugv8p9

[Mark Rutland <mark.rutland@arm.com>]

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
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.

[...]

>  .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

Mark.