Subject: [PATCHv5 15/16] ARM: hyp: initialize CNTVOFF to zero

From: [email-masked] (Dave Martin)
Date: Fri, 1 Feb 2013 11:13:50 +0000

On Thu, Jan 31, 2013 at 12:15:38PM +0000, Mark Rutland wrote:
> From: Marc Zyngier <[email-masked]>
> 
> In order to be able to use the virtual counter in a safe way,
> make sure it is initialized to zero before dropping to SVC.
> 
> Signed-off-by: Marc Zyngier <[email-masked]>
> Signed-off-by: Mark Rutland <[email-masked]>
> Cc: Dave Martin <[email-masked]>
> ---
>  arch/arm/kernel/hyp-stub.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm/kernel/hyp-stub.S b/arch/arm/kernel/hyp-stub.S
> index 65b2417..455603a 100644
> --- a/arch/arm/kernel/hyp-stub.S
> +++ b/arch/arm/kernel/hyp-stub.S
> @@ -152,6 +152,9 @@ THUMB(	orr	r7, #(1 << 30)	)	@ HSCTLR.TE
>  	mrc	p15, 4, r7, c14, c1, 0	@ CNTHCTL
>  	orr	r7, r7, #3		@ PL1PCEN | PL1PCTEN
>  	mcr	p15, 4, r7, c14, c1, 0	@ CNTHCTL
> +	mov	r6, #0
> +	mov	r7, #0
> +	mcrr	p15, 4, r6, r7, c14	@ CNTVOFF

Is this required for safety, or is it more a sanity feature?

The architected timer counters are supposed to be monotonic time sources
only, so applying a random offset shouldn't really change anything.

The main thing I can think of is that it is easier for the host to
manage guests' virtual counter offsets if the host's offset is 0 (and
we don't really want to be changing the host offset after the host kernel
boots).

Cheers
---Dave