Re: [bootwrapper PATCH] aarch64: Enable BRBE for the non-secure world

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

On Mon, Jan 17, 2022 at 04:06:44PM +0530, Anshuman Khandual wrote:
> On 1/13/22 3:54 PM, Mark Rutland wrote:
> > Hi Anshuman,
> > 
> > This looks fine structurally, I'm just not sure of a couple of details because
> > I can't find the relevant documentation -- more on that below.
> > 
> > On Thu, Jan 13, 2022 at 03:11:08PM +0530, Anshuman Khandual wrote:
> >> MDCR_EL3.SBRBE resets to an UNKNOWN value. Configure it to allow the BRBE
> >> buffer usage and direct register access in the non-secure world. But just
> >> before that, check AA64DFR0_EL1.BRBE and make sure BRBE is implemented. We
> >> still continue to reset MDCR_EL3 register to zero with the exception of
> >> MDCR_EL3.NSPB, MDCR_EL3.NSTB and MDCR_EL3.SBRBE.
> > 
> > I'm struggling to find where the BRBE system register fields are documented.
> > 
> > I looked at the latest ARM ARM (DDI 0487G.b):
> > 
> >   https://developer.arm.com/documentation/ddi0487/gb/
> > 
> > ... and the Armv9 supplement (DDI 0608A.a):
> > 
> >   https://developer.arm.com/documentation/ddi0608/aa/
> > 
> > ... but AFAICT, neither of those describe the bit-positions of the relevant
> > fields, so I can't check that those are correct. The other extensions (at leat
> > TME) describe that in the supplement, so this looks like a bug/oversight. 
> > 
> > Am I looking at the right documents? If this is meant to be in the supplement,
> > could you please raise a bug report to get that fixed?
> 
> Please find the MDCR_EL3.SBRBE definition here.
> 
> https://developer.arm.com/documentation/ddi0601/2021-12/AArch64-Registers/
> MDCR-EL3--Monitor-Debug-Configuration-Register--EL3-?lang=en#fieldset_0-33_32-1
> 
> > 
> >>
> >> 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 bfbb6ec..ca1b292 100644
> >> --- a/arch/aarch64/boot.S
> >> +++ b/arch/aarch64/boot.S
> >> @@ -103,6 +103,14 @@ ASM_FUNC(_start)
> >>  	ldr	x1, =(0x3 << 24)
> >>  	orr	x0, x0, x1
> >>  
> >> +1:	mrs     x1, id_aa64dfr0_el1
> >> +	ubfx    x1, x1, #52, #4
> >> +	cbz     x1, 1f
> >> +
> >> +	// Enable BRBE for the non-secure world.
> >> +	ldr     x1, =(0x3 << 32)
> >> +	orr     x0, x0, x1
> >> +
> > 
> > I assume this is the `SBRBE` field, which naming-wise sounds like it controls
> > Secure rather than Non-Secure (e,g. by way of comparison to `NSPB`). Is that
> 
> Right, that is some what counter-intuitive.
> 
> > correct? What effect does the value 0x3 have?
> 
> As per the definition above.
> 
> 0b11	 This control does not cause any direct accesses to BRBE registers or
>          instruction to be trapped, and does not cause any Exception levels to
> 	 be a prohibited region.
> 
> This basically allows BRBE usage in EL2/EL1.

Thanks for this!

Just to check, my understanding, there's nothing else that we need to
initialize to ensure that BRBE won't start recording unexpectedly
out-of-reset, becuase in BRBCR_EL* the E*BRE bits all warm-reset to 0
and hence prevent recording in any of EL0/EL1/EL2 (and BRBE never
records at EL3). Is that right?

Assuming so, I'll take this as-is.

Thanks,
Mark.

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