Re: [Xen-devel] [RFC PATCH v4 09/11] xen: arm: add cpufreq shared info definition

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 6 Nov 2014, Oleksandr Dmytryshyn wrote:
> On Tue, Nov 4, 2014 at 6:12 PM, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Tue, 4 Nov 2014, Oleksandr Dmytryshyn wrote:
> >> This shared info will be used by hwdom-cpufreq driver
> >> to send commands to the cpufreq driver in the hwdom.
> >>
> >> Signed-off-by: Oleksandr Dmytryshyn <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx>
> >> ---
> >>  xen/include/asm-arm/shared.h  | 14 ++++++++++++++
> >>  xen/include/public/arch-arm.h |  8 ++++++++
> >>  2 files changed, 22 insertions(+)
> >>  create mode 100644 xen/include/asm-arm/shared.h
> >>
> >> diff --git a/xen/include/asm-arm/shared.h b/xen/include/asm-arm/shared.h
> >> new file mode 100644
> >> index 0000000..4906f38
> >> --- /dev/null
> >> +++ b/xen/include/asm-arm/shared.h
> >> @@ -0,0 +1,14 @@
> >> +#ifndef __XEN_ARM_SHARED_H__
> >> +#define __XEN_ARM_SHARED_H__
> >> +
> >> +#define GET_SET_SHARED(type, field)                                 \
> >> +static inline type *arch_get_##field##_addr(const struct domain *d) \
> >> +{                                                                   \
> >> +   return &d->shared_info->arch.field;                              \
> >> +}
> >> +
> >> +GET_SET_SHARED(struct cpufreq_sh_info, cpufreq)
> >> +
> >> +#undef GET_SET_SHARED
> >> +
> >> +#endif /* __XEN_ARM_SHARED_H__ */
> >> diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
> >> index 7496556..7eef6f7 100644
> >> --- a/xen/include/public/arch-arm.h
> >> +++ b/xen/include/public/arch-arm.h
> >> @@ -309,7 +309,15 @@ struct arch_vcpu_info {
> >>  };
> >>  typedef struct arch_vcpu_info arch_vcpu_info_t;
> >>
> >> +struct cpufreq_sh_info {
> >> +    uint32_t cpu;       /* Physical CPU number */
> >> +    uint32_t freq;      /* Frequency in KHz */
> >> +    uint32_t relation;  /* CPUFREQ_RELATION_L/H (0) or (1) */
> >> +    int32_t result;        /* Returned result of the operation */
> >> +};
> >> +
> >>  struct arch_shared_info {
> >> +    struct cpufreq_sh_info cpufreq;
> >>  };
> >>  typedef struct arch_shared_info arch_shared_info_t;
> >>  typedef uint64_t xen_callback_t;
> >
> > This introduces one global struct cpufreq_sh_info. Do we need to worry
> > about locking? Is it possible that we might want to send two commands
> > simultaneously? How does Xen get to know that Dom0 completed the
> > previous operation before starting a new one?
> As I've written before:
> I'll create an event channel between Xen and hwdom.
> In this case Xen will be able to know that hwdom completed the
> previous operation before starting a new one.

I don't think that Xen can receive event channel notifications.  You
might have to introduce some kind of synchronization protocol within
struct cpufreq_sh_info.

It would greatly simplify your problem if you made sure that the guest
cpufreq "handler" only ran on one particular vcpu. Maybe you could
register the vcpu handler at boot time via hypercall. Otherwise you
could have one struct cpufreq_sh_info per guest vcpu.

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