Re: [PATCH v4] x86/xen: Add support for HVMOP_set_evtchn_upcall_vector

[Juergen Gross <jgross@xxxxxxxx>]

On 11.11.22 14:17, Jan Beulich wrote:
> On 11.11.2022 13:44, Juergen Gross wrote:
> > On 11.11.22 10:01, Juergen Gross wrote:
> > > On 08.11.22 17:26, Jan Beulich wrote:
> > > > On 03.11.2022 16:41, Jan Beulich wrote:
> > > > > On 03.11.2022 14:38, Jan Beulich wrote:
> > > > > > On 29.07.2022 09:04, Jane Malalane wrote:
> > > > > > > @@ -125,6 +130,9 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_xen_hvm_callback)
> > > > > > >    {
> > > > > > >        struct pt_regs *old_regs = set_irq_regs(regs);
> > > > > > > +    if (xen_percpu_upcall)
> > > > > > > +        ack_APIC_irq();
> > > > > > > +
> > > > > > >        inc_irq_stat(irq_hv_callback_count);
> > > > > > >        xen_hvm_evtchn_do_upcall();
> > > > > > > @@ -168,6 +176,15 @@ static int xen_cpu_up_prepare_hvm(unsigned int cpu)
> > > > > > >        if (!xen_have_vector_callback)
> > > > > > >            return 0;
> > > > > > > +    if (xen_percpu_upcall) {
> > > > > > > +        rc = xen_set_upcall_vector(cpu);
> > > > > >   From all I can tell at least for APs this happens before setup_local_apic().
> > > > > > With there being APIC interaction in this operation mode, as seen e.g. in
> > > > > > the earlier hunk above, I think this is logically wrong. And it leads to
> > > > > > apic_pending_intr_clear() issuing its warning: The vector registration, as
> > > > > > an intentional side effect, marks the vector as pending. Unless IRQs were
> > > > > > enabled at any point between the registration and the check, there's
> > > > > > simply no way for the corresponding IRR bit to be dealt with (by
> > > > > > propagating to ISR when the interrupt is delivered, and then being cleared
> > > > > > from ISR by EOI).
> > > > > With Roger's help I now have a pointer to osstest also exposing the issue:
> > > > >
> > > > > http://logs.test-lab.xenproject.org/osstest/logs/174592/test-amd64-amd64-xl-pvhv2-intel/huxelrebe0---var-log-xen-console-guest-debian.guest.osstest.log.gz
> > > > I've noticed only now that my mail to Jane bounced, and I'm now told
> > > > she's no longer in her role at Citrix. Since I don't expect to have time
> > > > to investigate an appropriate solution here, may I ask whether one of
> > > > the two of you could look into this, being the maintainers of this code?
> > > I think the correct way to handle this would be:
> > >
> > > - rename CPUHP_AP_ARM_XEN_STARTING to CPUHP_AP_XEN_STARTING
> > > - move the xen_set_upcall_vector() call to a new hotplug callback
> > >     registered for CPUHP_AP_XEN_STARTING (this can be done even
> > >     conditionally only if xen_percpu_upcall is set)
> > >
> > > Writing a patch now ...
> > For the APs this is working as expected.
> >
> > The boot processor seems to be harder to fix. The related message is being
> > issued even with interrupts being on when setup_local_APIC() is called.
> Hmm, puzzling: I don't recall having seen the message for the BSP. Which
> made me assume (without having actually checked) that ...
>
> > I've tried to register the callback only after the setup_local_APIC() call,
> ... it's already happening afterwards in that case.
>
> > but this results in a system hang when the APs are started.
> >
> > Any ideas?
> Not really, to be honest.
I might be wrong here, but is a bit set in IRR plus interrupts enabled
enough to make the kernel happy? The local APIC isn't enabled yet when
apic_pending_intr_clear() is being called, so IMHO the hypervisor will
never propagate the bit to ISR.

I didn't find any specific information in the SDM regarding "accepting
an interrupt" of a disabled local APIC, but maybe I didn't find the
relevant part of the manual.


Juergen

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