Re: [PATCH v6 09/11] arm/hvf: Add a WFI handler

[Peter Maydell <peter.maydell@linaro.org>]

On Wed, 10 Feb 2021 at 20:25, Peter Collingbourne <pcc@google.com> wrote:
>
> On Thu, Jan 28, 2021 at 8:25 AM Peter Maydell <peter.maydell@linaro.org> wrote:
> >
> > On Wed, 20 Jan 2021 at 22:44, Alexander Graf <agraf@csgraf.de> wrote:
> > > +                if (!seconds && nanos < 2000000) {
> > > +                    break;
> > > +                }
> > > +
> > > +                struct timespec ts = { seconds, nanos };
> > > +                hvf_wait_for_ipi(cpu, &ts);
> > > +            }
> >
> > Why doesn't the timer timeout manifest as an IPI ? (Put another way,
> > why is the timer interrupt special?)
>
> Timer timeouts result in an IPI (via HV_EXIT_REASON_VTIMER_ACTIVATED)
> if they become due while in hv_vcpu_run(). But at this point we are
> not in hv_vcpu_run() (due to the aforementioned difference in wait
> behavior between x86 and ARM) so we need to "manually" wait for the
> timer to become due, re-enter the guest, let it exit with
> HV_EXIT_REASON_VTIMER_ACTIVATED and then trigger the IPI.

But WFI doesn't just wait for a timer interrupt, it waits for
any interrupt. So it doesn't seem right that the timer interrupt
in particular is being handled specially here.

thanks
-- PMM