Re: NetBSD dom0 PVH: hardware interrupts stalls

[Jan Beulich <jbeulich@suse.com>]

On 20.11.2020 09:52, Manuel Bouyer wrote:
> On Fri, Nov 20, 2020 at 09:28:55AM +0100, Roger Pau Monné wrote:
>>> i.e. at that point we're waiting for Dom0 to signal it's done handling
>>> the IRQ. There is, however, a timer associated with this. Yet that's
>>> actually to prevent the system getting stuck, i.e. the "in-flight"
>>> state ought to clear 1ms later (when that timer expires), and hence
>>> ought to be pretty unlikely to catch when non-zero _and_ something's
>>> actually stuck.
>>
>> I somehow assumed the interrupt was in-flight because the printing to
>> the Xen console caused one to be injected, and thus dom0 didn't had
>> time to Ack it yet.
> 
> What does Xen consider to be an ACK from the dom0 ?
> AFAIK we have EOI only for LAPIC interrupts.

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.

Jan