On 16 April 2018 at 15:03, Alex Bennée <alex.ben...@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.ben...@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