Re: NetBSD dom0 PVH: hardware interrupts stalls

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.11.2020 10:27, Manuel Bouyer wrote:
> On Fri, Nov 20, 2020 at 09:59:57AM +0100, Jan Beulich wrote:
>> Well, anything coming through the LAPIC needs ack-ing (except for
>> the spurious interrupt of course), or else ISR won't get updated
>> and further interrupts at this or lower priority can't be serviced
>> (delivered) anymore. This includes interrupts originally coming
>> through the IO-APIC. But the same constraint / requirement exists
>> on baremetal.
> 
> OK, so even if I didn't see where this happens, it's happening.
> Is it what's Xen is using as ACK from the dom0 for a IOAPIC
> interrupt, or is it something else (at the IOAPIC level) ?

It's the traditional LAPIC based EOI mechanism that Xen intercepts
(as necessary) on the guest side and then translates (via
surprisingly many layers of calls) into the necessary EOI /
unmask / whatever at the hardware level. Our vIO-APIC
implementation so far doesn't support IO-APIC based EOI at all
(which is reflected in the IO-APIC version ID being 0x11).

Jan