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

[Marc Zyngier <maz@kernel.org>]

On Wed, 11 Aug 2021 14:55:58 +0100,
Mark Rutland <mark.rutland@arm.com> wrote:
> 
> 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!

I was just as surprised. PFR would have made a lot more sense, but I
guess someone wanted to use these last four bits as quickly as
possible... ;-)

> 
> > Signed-off-by: Marc Zyngier <maz@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`.

Good point. I'm surprised ECV3 hasn't hit yet! :D

> > +
> > +	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;

If you are actively rewriting this, maybe don't bother with the patch
and just add this as part of your rewrite.

> 
> Thanks,
> Mark.
> ---->8----
> From bc6a9380eb3c5afc96735a54d455f2487df48700 Mon Sep 17 00:00:00 2001
> From: Marc Zyngier <maz@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@kernel.org>
> [Mark: read id_aa64mmfr0_el1 separately, s/bne/b.lt/]
> Signed-off-by: Mark Rutland <mark.rutland@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

Otherwise looks fine to me.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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