Re: [Qemu-devel] [PATCH v3 12/22] target/arm: Filter cycle counter based on PMCCFILTR_EL0

[Aaron Lindsay <alindsay@codeaurora.org>]

On Apr 12 13:36, Aaron Lindsay wrote:
> On Apr 12 18:15, Peter Maydell wrote:
> > On 16 March 2018 at 20:31, Aaron Lindsay <alindsay@codeaurora.org> wrote:
> > > diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> > > index b0ef727..9c3b5ef 100644
> > > --- a/target/arm/cpu.h
> > > +++ b/target/arm/cpu.h
> > > @@ -458,6 +458,11 @@ typedef struct CPUARMState {
> > >           * was reset. Otherwise it stores the counter value
> > >           */
> > >          uint64_t c15_ccnt;
> > > +        /* ccnt_cached_cycles is used to hold the last cycle count when
> > > +         * c15_ccnt holds the guest-visible count instead of the delta during
> > > +         * PMU operations which require this.
> > > +         */
> > > +        uint64_t ccnt_cached_cycles;
> > 
> > Can this ever hold valid state at a point when we need to do VM
> > migration, or is it purely temporary ?
> 
> I believe that as of this version of the patch it is temporary and will
> not need to be migrated. However, I believe it's going to be necessary
> to have two variables to represent the state of each counter in order to
> implement interrupt on overflow. 

Coming back around to this, I don't see a way around using two variables
to hold PMCCNTR's full state to make interrupt on overflow work. I
haven't been able to find other examples or documentation covering state
needing to be updated in more than one location for a given CP register
- do you know of any I've missed or have recommendations about how to
approach this?

-Aaron

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