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

[Suzuki K Poulose]

On 2/12/21 2:31 PM, Alexandru Elisei wrote:
> Hello Anshuman,
> 
> 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 at 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
>> +	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
>>   
>>   	mrs	x0, id_aa64pfr0_el1
> 
> That's strange, I'm looking at ARM DDI 0487G.a and bits [44:47] from
> ID_AA64DFR0_EL1 are RES0 and there is no TraceBuffer field; bits [24:25] of
> MDCR_EL3 are also RES0 and I searched the entire file for the NSTB field, could
> not find it. Do I have an outdated version of the architecture?

They are not in the Arm ARM. These are part of the Future Architecture
technology changes, for which the register defintions are available here :

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

Cheers
Suzuki

> 
> Thanks,
> 
> Alex
>