Re: [RFC PATCH v3 4/6] psci: Add hvc call service for ptp_kvm.

[Paolo Bonzini]

On 09/10/19 18:05, John Stultz wrote:
> On Wed, Oct 9, 2019 at 2:13 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> John (Stultz), does that sound good to you?  The context is that
>> Jianyong would like to add a hypercall that returns a (cycles,
>> nanoseconds) pair to the guest.  On x86 we're relying on the vclock_mode
>> field that is already there for the vDSO, but being able to just use
>> ktime_get_snapshot would be much nicer.
> 
> I've not really looked at the code closely in awhile, so I'm not sure
> my suggestions will be too useful.
> 
> My only instinct is maybe to not include the clocksource pointer in
> the system_time_snapshot, as I worry that structure will then be
> abused by the interface users.  If you're just wanting to make sure
> the clocksource is what you're expecting, would instead putting only
> the clocksource name in the structure suffice?

Well, it would suffice but it would be quite ugly to do a string
comparison later.

What kind of abuse are you thinking of?  We already have struct
system_counterval_t for a clocksource+cycles tuple, so it seemed obvious
to me to make system_time_snapshot a superset of it...  In fact,
system_time_snapshot's cycles member is even unused currently, so it
could even be easily replaced by a struct system_counterval_t, instead
of adding an extra field.

Paolo