Re: [Qemu-devel] [RFC PATCH] target/arm: support reading of CNTVCT_EL0 from user-space

[Peter Maydell <peter.maydell@linaro.org>]

On 16 April 2018 at 15:03, Alex Bennée <alex.bennee@linaro.org> wrote:
> Since kernel commit a86bd139f2 (arm64: arch_timer: Enable CNTVCT_EL0
> trap..) user-space has been able to read this system register. This
> patch enables access to that register although currently it always
> returns 0 as we don't yet have a mechanism for managing timers in
> linux-user mode.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  target/arm/helper.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index b14fdab140..8244badd63 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -2121,11 +2121,25 @@ static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
>  };
>
>  #else
> -/* In user-mode none of the generic timer registers are accessible,
> - * and their implementation depends on QEMU_CLOCK_VIRTUAL and qdev gpio outputs,
> - * so instead just don't register any of them.
> +
> +/* In user-mode most of the generic timer registers are inaccessible
> + * however modern kernels (4.12+) allow access to cntvct_el0
>   */
> +
> +static uint64_t gt_virt_cnt_read(CPUARMState *env, const ARMCPRegInfo *ri)
> +{
> +    /* Currently we have no support for QEMUTimer in linux-user so we
> +     * can't call gt_get_countervalue(env).
> +     */
> +    return 0;
> +}
> +
>  static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
> +    { .name = "CNTVCT_EL0", .state = ARM_CP_STATE_AA64,
> +      .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 2,
> +      .access = PL0_R, .type = ARM_CP_NO_RAW | ARM_CP_IO,
> +      .readfn = gt_virt_cnt_read,
> +    },
>      REGINFO_SENTINEL
>  };

CNTVCT_EL0 isn't much use without CNTFRQ_EL0 which tells
you how fast it ticks...

It looks like other targets use cpu_get_host_ticks() for an
arbitrary time-counter thingy. Not sure you can get the frequency
for it, though :-(

thanks
-- PMM