Re: [PATCH v19 09/11] arm64: Handle BRBE booting requirements

[Anshuman Khandual]

On 2/3/25 06:13, Rob Herring (Arm) wrote:
> From: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> 
> To use the Branch Record Buffer Extension (BRBE), some configuration is
> necessary at EL3 and EL2. This patch documents the requirements and adds
> the initial EL2 setup code, which largely consists of configuring the
> fine-grained traps and initializing a couple of BRBE control registers.
> 
> Before this patch, __init_el2_fgt() would initialize HDFGRTR_EL2 and
> HDFGWTR_EL2 with the same value, relying on the read/write trap controls
> for a register occupying the same bit position in either register. The
> 'nBRBIDR' trap control only exists in bit 59 of HDFGRTR_EL2, while bit
> 59 of HDFGRTR_EL2 is RES0, and so this assumption no longer holds.
> 
> To handle HDFGRTR_EL2 and HDFGWTR_EL2 having (slightly) different bit
> layouts, __init_el2_fgt() is changed to accumulate the HDFGRTR_EL2 and
> HDFGWTR_EL2 control bits separately. While making this change the
> open-coded value (1 << 62) is replaced with
> HDFG{R,W}TR_EL2_nPMSNEVFR_EL1_MASK.
> 
> The BRBCR_EL1 and BRBCR_EL2 registers are unusual and require special
> initialisation: even though they are subject to E2H renaming, both have
> an effect regardless of HCR_EL2.TGE, even when running at EL2, and
> consequently both need to be initialised. This is handled in
> __init_el2_brbe() with a comment to explain the situation.
> 
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> [Mark: rewrite commit message, fix typo in comment]

This commit message after rewrite is better, thanks !

> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Rob Herring (Arm) <robh@xxxxxxxxxx>
> ---
>  Documentation/arch/arm64/booting.rst | 21 +++++++++
>  arch/arm64/include/asm/el2_setup.h   | 86 ++++++++++++++++++++++++++++++++++--
>  2 files changed, 104 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/arch/arm64/booting.rst b/Documentation/arch/arm64/booting.rst
> index cad6fdc96b98..0a421757cacf 100644
> --- a/Documentation/arch/arm64/booting.rst
> +++ b/Documentation/arch/arm64/booting.rst
> @@ -352,6 +352,27 @@ Before jumping into the kernel, the following conditions must be met:
>  
>      - HWFGWTR_EL2.nSMPRI_EL1 (bit 54) must be initialised to 0b01.
>  
> +  For CPUs with feature Branch Record Buffer Extension (FEAT_BRBE):
> +
> +  - If EL3 is present:
> +
> +    - MDCR_EL3.SBRBE (bits 33:32) must be initialised to 0b11.
> +
> +  - If the kernel is entered at EL1 and EL2 is present:
> +
> +    - BRBCR_EL2.CC (bit 3) must be initialised to 0b1.
> +    - BRBCR_EL2.MPRED (bit 4) must be initialised to 0b1.
> +
> +    - HDFGRTR_EL2.nBRBDATA (bit 61) must be initialised to 0b1.
> +    - HDFGRTR_EL2.nBRBCTL  (bit 60) must be initialised to 0b1.
> +    - HDFGRTR_EL2.nBRBIDR  (bit 59) must be initialised to 0b1.
> +
> +    - HDFGWTR_EL2.nBRBDATA (bit 61) must be initialised to 0b1.
> +    - HDFGWTR_EL2.nBRBCTL  (bit 60) must be initialised to 0b1.
> +
> +    - HFGITR_EL2.nBRBIALL (bit 56) must be initialised to 0b1.
> +    - HFGITR_EL2.nBRBINJ  (bit 55) must be initialised to 0b1.
> +
>    For CPUs with the Scalable Matrix Extension FA64 feature (FEAT_SME_FA64):
>  
>    - If EL3 is present:
> diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> index 25e162651750..bf21ce513aff 100644
> --- a/arch/arm64/include/asm/el2_setup.h
> +++ b/arch/arm64/include/asm/el2_setup.h
> @@ -163,6 +163,39 @@
>  .Lskip_set_cptr_\@:
>  .endm
>  
> +/*
> + * Configure BRBE to permit recording cycle counts and branch mispredicts.
> + *
> + * At any EL, to record cycle counts BRBE requires that both BRBCR_EL2.CC=1 and
> + * BRBCR_EL1.CC=1.
> + *
> + * At any EL, to record branch mispredicts BRBE requires that both
> + * BRBCR_EL2.MPRED=1 and BRBCR_EL1.MPRED=1.
> + *
> + * When HCR_EL2.E2H=1, the BRBCR_EL1 encoding is redirected to BRBCR_EL2, but
> + * the {CC,MPRED} bits in the real BRBCR_EL1 register still apply.
> + *
> + * Set {CC,MPRED} in both BRBCR_EL2 and BRBCR_EL1 so that at runtime we only
> + * need to enable/disable these in BRBCR_EL1 regardless of whether the kernel
> + * ends up executing in EL1 or EL2.
> + */
> +.macro __init_el2_brbe
> +	mrs	x1, id_aa64dfr0_el1
> +	ubfx	x1, x1, #ID_AA64DFR0_EL1_BRBE_SHIFT, #4
> +	cbz	x1, .Lskip_brbe_\@
> +
> +	mov_q	x0, BRBCR_ELx_CC | BRBCR_ELx_MPRED
> +	msr_s	SYS_BRBCR_EL2, x0
> +
> +	__check_hvhe .Lset_brbe_nvhe_\@, x1
> +	msr_s	SYS_BRBCR_EL12, x0	// VHE
> +	b	.Lskip_brbe_\@
> +
> +.Lset_brbe_nvhe_\@:
> +	msr_s	SYS_BRBCR_EL1, x0	// NVHE
> +.Lskip_brbe_\@:
> +.endm
> +
>  /* Disable any fine grained traps */
>  .macro __init_el2_fgt
>  	mrs	x1, id_aa64mmfr0_el1
> @@ -170,16 +203,48 @@
>  	cbz	x1, .Lskip_fgt_\@
>  
>  	mov	x0, xzr
> +	mov	x2, xzr
>  	mrs	x1, id_aa64dfr0_el1
>  	ubfx	x1, x1, #ID_AA64DFR0_EL1_PMSVer_SHIFT, #4
>  	cmp	x1, #3
>  	b.lt	.Lskip_spe_fgt_\@
> +
>  	/* Disable PMSNEVFR_EL1 read and write traps */
> -	orr	x0, x0, #(1 << 62)
> +	orr	x0, x0, #HDFGRTR_EL2_nPMSNEVFR_EL1_MASK
> +	orr	x2, x2, #HDFGWTR_EL2_nPMSNEVFR_EL1_MASK
>  
>  .Lskip_spe_fgt_\@:
> +#ifdef CONFIG_ARM64_BRBE
> +	mrs	x1, id_aa64dfr0_el1
> +	ubfx	x1, x1, #ID_AA64DFR0_EL1_BRBE_SHIFT, #4
> +	cbz	x1, .Lskip_brbe_reg_fgt_\@
> +
> +	/*
> +	 * Disable read traps for the following registers
> +	 *
> +	 * [BRBSRC|BRBTGT|RBINF]_EL1
> +	 * [BRBSRCINJ|BRBTGTINJ|BRBINFINJ|BRBTS]_EL1
> +	 */
> +	orr	x0, x0, #HDFGRTR_EL2_nBRBDATA_MASK
> +
> +	/*
> +	 * Disable write traps for the following registers
> +	 *
> +	 * [BRBSRCINJ|BRBTGTINJ|BRBINFINJ|BRBTS]_EL1
> +	 */
> +	orr	x2, x2, #HDFGWTR_EL2_nBRBDATA_MASK
> +
> +	/* Disable read and write traps for [BRBCR|BRBFCR]_EL1 */
> +	orr	x0, x0, #HDFGRTR_EL2_nBRBCTL_MASK
> +	orr	x2, x2, #HDFGWTR_EL2_nBRBCTL_MASK
> +
> +	/* Disable read traps for BRBIDR_EL1 */
> +	orr	x0, x0, #HDFGRTR_EL2_nBRBIDR_MASK
> +
> +.Lskip_brbe_reg_fgt_\@:
> +#endif /* CONFIG_ARM64_BRBE */
>  	msr_s	SYS_HDFGRTR_EL2, x0
> -	msr_s	SYS_HDFGWTR_EL2, x0
> +	msr_s	SYS_HDFGWTR_EL2, x2
>  
>  	mov	x0, xzr
>  	mrs	x1, id_aa64pfr1_el1
> @@ -220,7 +285,21 @@
>  .Lset_fgt_\@:
>  	msr_s	SYS_HFGRTR_EL2, x0
>  	msr_s	SYS_HFGWTR_EL2, x0
> -	msr_s	SYS_HFGITR_EL2, xzr
> +	mov	x0, xzr
> +#ifdef CONFIG_ARM64_BRBE
> +	mrs	x1, id_aa64dfr0_el1
> +	ubfx	x1, x1, #ID_AA64DFR0_EL1_BRBE_SHIFT, #4
> +	cbz	x1, .Lskip_brbe_insn_fgt_\@
> +
> +	/* Disable traps for BRBIALL instruction */
> +	orr	x0, x0, #HFGITR_EL2_nBRBIALL_MASK
> +
> +	/* Disable traps for BRBINJ instruction */
> +	orr	x0, x0, #HFGITR_EL2_nBRBINJ_MASK
> +
> +.Lskip_brbe_insn_fgt_\@:
> +#endif /* CONFIG_ARM64_BRBE */
> +	msr_s	SYS_HFGITR_EL2, x0
>  
>  	mrs	x1, id_aa64pfr0_el1		// AMU traps UNDEF without AMU
>  	ubfx	x1, x1, #ID_AA64PFR0_EL1_AMU_SHIFT, #4
> @@ -275,6 +354,7 @@
>  	__init_el2_hcrx
>  	__init_el2_timers
>  	__init_el2_debug
> +	__init_el2_brbe
>  	__init_el2_lor
>  	__init_el2_stage2
>  	__init_el2_gicv3
> 

LGTM.

Both commit message and in code comments formatting have been changed from V18.