Re: [boot-wrapper] [PATCH] aarch64: Enable TRBE for the non-secure world

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hello,

On 2/11/21 11:26 AM, Anshuman Khandual wrote:
> MDCR_EL3.NSTB resets to an UNKNOWN value. Configure it to allow the trace
> buffer to use non-secure memory and to permit direct register accesses from
> the non-secure world. Before that, just check AA64DFR0_EL1.TraceBuffer and
> make sure TRBE is implemented. We still continue to reset MDCR_EL3 register
> to zero with the exception of MDCR_EL3.NSPB and MDCR_EL3.NSTB.
>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  arch/aarch64/boot.S | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/aarch64/boot.S b/arch/aarch64/boot.S
> index 37f4b98..e47cf59 100644
> --- a/arch/aarch64/boot.S
> +++ b/arch/aarch64/boot.S
> @@ -71,6 +71,14 @@ _start:
>  	ldr	x1, =(0x3 << 12)
>  	orr	x0, x0, x1
>  
> +1:	mrs	x1, id_aa64dfr0_el1

Nitpick: we could change the first read of ID_AA64DFR0_EL1 to use x2 as the
destination register, to avoid this second read.

> +	ubfx	x1, x1, #44, #4
> +	cbz	x1, 1f
> +
> +	// Enable TRBE for the non-secure world.
> +	ldr	x1, =(0x3 << 24)
> +	orr	x0, x0, x1
> +
>  1:	msr	mdcr_el3, x0			// Disable traps to EL3

Looked at [1] for the field definitions, and the patch indeed does what it says.
0b11 for MDCR_EL3.NSTB means that the buffer owning regime is the non-secure
state, and accesses to the buffer control registers from the *secure state* are
trapped to EL3, which is what we want.

With or without the destination register change:

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

[1] https://developer.arm.com/documentation/ddi0601/2020-12/AArch64-Registers/

Thanks,

Alex

>  
>  	mrs	x0, id_aa64pfr0_el1

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