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

[Alexander Graf]

On 10.02.21 23:17, Peter Maydell wrote:
> 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.


It waits for either an external interrupt (vcpu_kick() -> IPI -> signal 
-> pselect exits) or a vtimer (kept in the CPU thread, handled by hvf 
natively when vCPU is running, handled through the pselect timeout when 
in WFI mode).

In hvf on ARM, the vtimer is handled specially. It is owned by the 
kernel code when we're inside the CPU loop. We don't even get vtimer MSR 
exits. However when user space decides to not return to the kernel CPU 
loop, it needs to handle the vtimer expiry itself. We really only have 
that case on WFI.


Alex