Re: [PATCH V17 4/9] arm64/boot: Enable EL2 requirements for BRBE

[Anshuman Khandual]

On 5/29/24 16:21, Mark Rutland wrote:
> On Fri, Apr 05, 2024 at 08:16:34AM +0530, Anshuman Khandual wrote:
>> Fine grained trap control for BRBE registers, and instructions access need
>> to be configured in HDFGRTR_EL2, HDFGWTR_EL2 and HFGITR_EL2 registers when
>> kernel enters at EL1 but EL2 is present. This changes __init_el2_fgt() as
>> required.
>>
>> Similarly cycle and mis-prediction capture need to be enabled in BRBCR_EL1
>> and BRBCR_EL2 when the kernel enters either into EL1 or EL2. This adds new
>> __init_el2_brbe() to achieve this objective.
>>
>> This also updates Documentation/arch/arm64/booting.rst with all the above
>> EL2 along with MDRC_EL3.SBRBE requirements.
>>
>> First this replaces an existing hard encoding (1 << 62) with corresponding
>> applicable macro HDFGRTR_EL2_nPMSNEVFR_EL1_MASK.
>>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Jonathan Corbet <corbet@xxxxxxx>
>> Cc: Marc Zyngier <maz@xxxxxxxxxx>
>> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-doc@xxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ----
>> Changes in V17:
>>
>> - New patch added in the series
>> - Separated out from the BRBE driver implementation patch
>> - Dropped the comment in __init_el2_brbe()
>> - Updated __init_el2_brbe() with BRBCR_EL2.MPRED requirements
>> - Updated __init_el2_brbe() with __check_hvhe() constructs
>> - Updated booting.rst regarding MPRED, MDCR_EL3 and fine grained control
>>
>>  Documentation/arch/arm64/booting.rst | 26 ++++++++
>>  arch/arm64/include/asm/el2_setup.h   | 90 +++++++++++++++++++++++++++-
>>  2 files changed, 113 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/arch/arm64/booting.rst b/Documentation/arch/arm64/booting.rst
>> index b57776a68f15..512210da7dd2 100644
>> --- a/Documentation/arch/arm64/booting.rst
>> +++ b/Documentation/arch/arm64/booting.rst
>> @@ -349,6 +349,32 @@ 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 the kernel is entered at EL2 and EL1 is present:
>> +
>> +    - BRBCR_EL1.CC (bit 3) must be initialised to 0b1.
>> +    - BRBCR_EL1.MPRED (bit 4) must be initialised to 0b1.
> 
> IIUC this isn't necessary; if the kernel is entered at EL2, it's capable
> of initializing the EL1 regs, and it doesn't look like this silently
> affects something we'd need in the absence of a BRBE driver.

No, this does not affect anything other than the BRBE driver.

> 
> AFAICT the __init_el2_brbe() code you add below handles this, so I think
> this is redundant and can be deleted.

Did not understand the above. __init_el2_brbe() handles setting both BRBCR_EL2
and BRBCR_EL1 for CC and MPRED config irrespective of whether the kernel enters
EL2 directly or enters EL1 via EL2. But should not that be documented here for
both those scenarios ? OR because once the kernel is in EL2, it can configure
EL1 as required, so it is not a booting requirement anymore ?

> 
>> +  - 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.
>> +
>> +  - If EL3 is present:
>> +
>> +    - MDCR_EL3.SBRBE (bits 33:32) must be initialised to 0b11.
> 
> Minor nit: please list the EL3 requirements first, that way this can be
> read in EL3->EL2->EL1 order to match the FW boot-flow order.

Makes sense, will change the order.

> 
>> +
>>    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 b7afaa026842..7c12a8e658d4 100644
>> --- a/arch/arm64/include/asm/el2_setup.h
>> +++ b/arch/arm64/include/asm/el2_setup.h
>> @@ -154,6 +154,41 @@
>>  .Lskip_set_cptr_\@:
>>  .endm
>>  
>> +#ifdef CONFIG_ARM64_BRBE
>> +/*
>> + * Enable BRBE cycle count and miss-prediction
>> + *
>> + * BRBE requires both BRBCR_EL1.CC and BRBCR_EL2.CC fields, be set
>> + * for the cycle counts to be available in BRBINF<N>_EL1.CC during
>> + * branch record processing after a PMU interrupt. This enables CC
>> + * field on both these registers while still executing inside EL2.
> 
> Huh, it's a bit of an oddity to do that for a register that gets the E2H
> treatment, but that is what the ARM ARM says, looking at the pseudocode
> in ARM DDI 0487K.a:
> 
> | // BRBCycleCountingEnabled()
> | // =========================
> | // Returns TRUE if the recording of cycle counts is allowed,
> | // FALSE otherwise.
> | boolean BRBCycleCountingEnabled()
> | 	if HaveEL(EL2) && BRBCR_EL2.CC == '0' then return FALSE;
> | 	if BRBCR_EL1.CC == '0' then return FALSE;
> | 	return TRUE;
> 
> ... and likewise for MPRED:
> 
> | // BRBEMispredictAllowed()
> | // =======================
> | // Returns TRUE if the recording of branch misprediction is allowed,
> | // FALSE otherwise.
> | boolean BRBEMispredictAllowed()
> | 	if HaveEL(EL2) && BRBCR_EL2.MPRED == '0' then return FALSE;
> | 	if BRBCR_EL1.MPRED == '0' then return FALSE;
> | 	return TRUE;
> 
> ... though BRBCycleCountingEnabled() isn't actually used anywhere, while
> BRBEMispredictAllowed is used by BRBEBranch(), since that does:
> 
> | (ccu, cc) = BranchEncCycleCount();
> | ...
> | bit mispredict = if BRBEMispredictAllowed() && BranchMispredict() then '1' else '0';
> 
> ... where BranchEncCycleCount() is a stub that doesn't mention
> BRBCycleCountingEnabled() at all, so it's not clear to me whether CCU is
> guaranteed to be set.
> 
>> + *
>> + * BRBE driver would still be able to toggle branch records cycle
>> + * count support via BRBCR_EL1.CC field regardless of whether the
>> + * kernel ends up executing in EL1 or EL2.
>> + *
>> + * The same principle applies for branch record mis-prediction info
>> + * as well, thus requiring MPRED field to be set on both BRBCR_EL1
>> + * and BRBCR_EL2 while still executing inside EL2.
>> + */
> 
> I think we can clarify this comment to:
> 
> /*
>  * Enable BRBE to record 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,MPRBED} in both BRBCR_EL2 and BRBCR_EL1 so that at runtime we
>  * only need to enable/disable thse in BRBCR_EL1 regardless of whether
>  * the kernel ends up executing in EL1 or EL2.
>  */

Sure, will update this comment.

> 
>> +.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
>> +#endif /* CONFIG_ARM64_BRBE */
>> +
>>  /* Disable any fine grained traps */
>>  .macro __init_el2_fgt
>>  	mrs	x1, id_aa64mmfr0_el1
>> @@ -161,16 +196,48 @@
>>  	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	.Lset_debug_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
>>  
>>  .Lset_debug_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
>> @@ -193,7 +260,21 @@
>>  .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
>> @@ -228,6 +309,9 @@
>>  	__init_el2_nvhe_idregs
>>  	__init_el2_cptr
>>  	__init_el2_fgt
>> +#ifdef CONFIG_ARM64_BRBE
>> +	__init_el2_brbe
>> +#endif
> 
> This largely looks fine, but I note that we haven't bothered with
> ifdeffery for PMU and SPE, so I suspect it might be worth getting rid of
> the ifdeffery for BRBE.

Sure, will drop the #ifdef

> 
> Mark.