[PATCH] arm64: initialize all of CNTHCTL_EL2

[Oliver Upton]

On Wed, Aug 18, 2021 at 9:15 AM Mark Rutland <mark.rutland at arm.com> wrote:
>
> In __init_el2_timers we initialize CNTHCTL_EL2.{EL1PCEN,EL1PCTEN} with a
> RMW sequence, leaving all other bits UNKNOWN.
>
> In general, we should initialize all bits in a register rather than
> using an RMW sequence, since most bits are UNKNOWN out of reset, and as
> new bits are added to the reigster their reset value might not result in
> expected behaviour.
>
> In the case of CNTHCTL_EL2, FEAT_ECV added a number of new control bits
> in previously RES0 bits, which reset to UNKNOWN values, and may cause
> issues for EL1 and EL0:
>
> * CNTHCTL_EL2.ECV enables the CNTPOFF_EL2 offset (which itself resets to
>   an UNKNOWN value) at EL0 and EL1. Since the offset could reset to
>   distinct values across CPUs, when the control bit resets to 1 this
>   could break timekeeping generally.
>
> * CNTHCTL_EL2.{EL1TVT,EL1TVCT} trap EL0 and EL1 accesses to the EL1
>   virtual timer/counter registers to EL2. When reset to 1, this could
>   cause unexpected traps to EL2.
>
> Initializing these bits to zero avoids these problems, and all other
> bits in CNPTOFF_EL2 other than EL1PCEN and EL1PCTEN can safely be reset
> to zero.

s/CNPTOFF_EL2/CNTHCTL_EL2

>
> This patch ensures we initialize CNTHCTL_EL2 accordingly, only setting
> EL1PCEN and EL1PCTEN, and setting all other bits to zero.
>
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Oliver Upton <oupton at google.com>
> Cc: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/include/asm/el2_setup.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> index 21fa330f498dd..b83fb24954b77 100644
> --- a/arch/arm64/include/asm/el2_setup.h
> +++ b/arch/arm64/include/asm/el2_setup.h
> @@ -33,8 +33,7 @@
>   * EL2.
>   */
>  .macro __init_el2_timers
> -       mrs     x0, cnthctl_el2
> -       orr     x0, x0, #3                      // Enable EL1 physical timers
> +       mov     x0, #3                          // Enable EL1 physical timers
>         msr     cnthctl_el2, x0
>         msr     cntvoff_el2, xzr                // Clear virtual offset
>  .endm
> --
> 2.11.0
>

Besides noted above:

Reviewed-by: Oliver Upton <oupton at google.com>