[PATCH][boot-wrapper] aarch64: Enable ECV to allow access to CNTPOFF_EL2

[Mark Rutland]

On Wed, Aug 11, 2021 at 10:22:26AM +0100, Marc Zyngier wrote:
> If the implemnentation supports ID_AA64MMFR0_EL1.ECV==2,
> set SCR_EL3.ECVEn to allow EL2 access to CNTPOFF_EL2.

I was about to ask if that was a typo (why is this in an MMFR?), but
that is what the ARM ARM says!

> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/aarch64/boot.S | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/aarch64/boot.S b/arch/aarch64/boot.S
> index 7f208b5..f0aa3cb 100644
> --- a/arch/aarch64/boot.S
> +++ b/arch/aarch64/boot.S
> @@ -54,10 +54,17 @@ ASM_FUNC(_start)
>  1:
>  	/* Enable FGT if present */
>  	mrs	x1, id_aa64mmfr0_el1
> -	ubfx	x1, x1, #56, #4
> -	cbz	x1, 1f
> +	ubfx	x2, x1, #56, #4
> +	cbz	x2, 1f
>  
>  	orr	x0, x0, #(1 << 27)		// FGT enable
> +1:
> +	/* Enable ECV2 if present (allows CNTPOFF_EL2) */
> +	ubfx	x2, x1, #60, #4
> +	cmp	x2, #2
> +	bne	1f

We need to check ID_AA64MMFR0_EL1.ECV >= 2 (to handle any futrue
variants) so the conditional branch needs to be something like `b.lt`.

> +
> +	orr	x0, x0, #(1 << 28)		// ECV enable

Aside from the above, this looks good to me, and I plan to apply this
with the below tweaks (updated patch below):

* Use b.lt, as above

* To keep each check self-contained, the ECV check will read
  id_aa64mmfr0_el1 itself. That removes the need to keep x1 around, and
  so we can leave the FGT check as-is.

  As part of some cleanup, I'm planning to move this to C, where feature
  checks will read ID fields via a helepr that implicitly reads the
  register, like:

  if (read_reg_field(ID_AA64MMFR0_EL1, ECV) >= 2)
  	scr |= SCR_EL3_ECVEN;

Thanks,
Mark.
---->8----
>From bc6a9380eb3c5afc96735a54d455f2487df48700 Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz at kernel.org>
Date: Wed, 11 Aug 2021 10:22:26 +0100
Subject: [PATCH] aarch64: Enable ECV to allow access to CNTPOFF_EL2

If the implemnentation supports ID_AA64MMFR0_EL1.ECV==2,
set SCR_EL3.ECVEn to allow EL2 access to CNTPOFF_EL2.

Signed-off-by: Marc Zyngier <maz at kernel.org>
[Mark: read id_aa64mmfr0_el1 separately, s/bne/b.lt/]
Signed-off-by: Mark Rutland <mark.rutland 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 7f208b5..2215f7e 100644
--- a/arch/aarch64/boot.S
+++ b/arch/aarch64/boot.S
@@ -59,6 +59,14 @@ ASM_FUNC(_start)
 
 	orr	x0, x0, #(1 << 27)		// FGT enable
 1:
+	/* Enable ECV2 if present (allows CNTPOFF_EL2) */
+	mrs	x1, id_aa64mmfr0_el1
+	ubfx	x1, x1, #60, #4
+	cmp	x1, #2
+	b.lt	1f
+
+	orr	x0, x0, #(1 << 28)		// ECV enable
+1:
 	/* Enable MTE if present */
 	mrs	x10, id_aa64pfr1_el1
 	ubfx	x10, x10, #8, #4
-- 
2.11.0