[v1,2/5] timer: arm: Introduce functions to get the host cntfrq

When acceleration like KVM is in use it's necessary to use the host's
counter frequency when converting ticks to or from time units.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
---
 include/qemu/timer.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

On Wed, 16 Oct 2019 at 15:34, Andrew Jones <drjones@redhat.com> wrote:
>
> When acceleration like KVM is in use it's necessary to use the host's
> counter frequency when converting ticks to or from time units.
>
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/qemu/timer.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/include/qemu/timer.h b/include/qemu/timer.h
> index 85bc6eb00b21..8941ddea8242 100644
> --- a/include/qemu/timer.h
> +++ b/include/qemu/timer.h
> @@ -1006,6 +1006,22 @@ static inline int64_t cpu_get_host_ticks(void)
>  }
>  #endif
>
> +#if defined(__aarch64__)
> +static inline uint32_t cpu_get_host_tick_frequency(void)
> +{
> +    uint64_t frq;
> +    asm volatile("mrs %0, cntfrq_el0" : "=r" (frq));
> +    return frq;
> +}
> +#elif defined(__arm__)
> +static inline uint32_t cpu_get_host_tick_frequency(void)
> +{
> +    uint32_t frq;
> +    asm volatile("mrc p15, 0, %0, c14, c0, 0" : "=r" (frq));
> +    return frq;
> +}
> +#endif

Don't we want to know what the guest counter frequency
is, not the host counter frequency? That is, I would have
expected that we get this value via doing a KVM ONE_REG ioctl
to ask the kernel what the guest cntfrq is. Are we using
the host value on the assumption that the guest might have
misprogrammed their copy of the register?

thanks
-- PMM

On Tue, Dec 10, 2019 at 03:47:39PM +0000, Peter Maydell wrote:
> On Wed, 16 Oct 2019 at 15:34, Andrew Jones <drjones@redhat.com> wrote:
> >
> > When acceleration like KVM is in use it's necessary to use the host's
> > counter frequency when converting ticks to or from time units.
> >
> > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> > ---
> >  include/qemu/timer.h | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/include/qemu/timer.h b/include/qemu/timer.h
> > index 85bc6eb00b21..8941ddea8242 100644
> > --- a/include/qemu/timer.h
> > +++ b/include/qemu/timer.h
> > @@ -1006,6 +1006,22 @@ static inline int64_t cpu_get_host_ticks(void)
> >  }
> >  #endif
> >
> > +#if defined(__aarch64__)
> > +static inline uint32_t cpu_get_host_tick_frequency(void)
> > +{
> > +    uint64_t frq;
> > +    asm volatile("mrs %0, cntfrq_el0" : "=r" (frq));
> > +    return frq;
> > +}
> > +#elif defined(__arm__)
> > +static inline uint32_t cpu_get_host_tick_frequency(void)
> > +{
> > +    uint32_t frq;
> > +    asm volatile("mrc p15, 0, %0, c14, c0, 0" : "=r" (frq));
> > +    return frq;
> > +}
> > +#endif
> 
> Don't we want to know what the guest counter frequency
> is, not the host counter frequency? That is, I would have
> expected that we get this value via doing a KVM ONE_REG ioctl
> to ask the kernel what the guest cntfrq is. Are we using
> the host value on the assumption that the guest might have
> misprogrammed their copy of the register?
>

Indeed it would be best to get whatever KVM is using for the given VCPU's
CNTFRQ through GET_ONE_REG, but KVM doesn't seem to allow it. I hadn't
tested before, but to be sure, I did now with the following kselftests
test

 #include "kvm_util.h"
 #include "processor.h"
 
 static void guest_code(void) { }
 
 int main(void)
 {
   struct kvm_vm *vm = vm_create_default(0, 0, guest_code);
   uint64_t reg;
 
   get_reg(vm, 0, ARM64_SYS_REG(3, 3, 14, 0, 0), &reg);
   printf("%lx\n", reg);
   return 0;
 }

The vcpu ioctl fails with ENOENT. Currently KVM requires all guests to
have the same frequency as the host, but I guess that will change
eventually. It's definitely best to do the save/restore thing, as you
suggest.

Thanks,
drew