Re: [PATCH] x86/HVM: don't mark evtchn upcall vector as pending when vLAPIC is disabled

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Nov 18, 2022 at 01:54:33PM +0100, Jan Beulich wrote:
> On 18.11.2022 13:33, Andrew Cooper wrote:
> > On 18/11/2022 10:31, Jan Beulich wrote:
> >> Linux'es relatively new use of HVMOP_set_evtchn_upcall_vector has
> >> exposed a problem with the marking of the respective vector as
> >> pending: For quite some time Linux has been checking whether any stale
> >> ISR or IRR bits would still be set while preparing the LAPIC for use.
> >> This check is now triggering on the upcall vector, as the registration,
> >> at least for APs, happens before the LAPIC is actually enabled.
> >>
> >> In software-disabled state an LAPIC would not accept any interrupt
> >> requests and hence no IRR bit would newly become set while in this
> >> state. As a result it is also wrong for us to mark the upcall vector as
> >> having a pending request when the vLAPIC is in this state.
> > 
> > I agree with this.
> > 
> >> To compensate for the "enabled" check added to the assertion logic, add
> >> logic to (conditionally) mark the upcall vector as having a request
> >> pending at the time the LAPIC is being software-enabled by the guest.
> > 
> > But this, I don't think is appropriate.
> > 
> > The point of raising on enable is allegedly to work around setup race
> > conditions.  I'm unconvinced by this reasoning, but it is what it is,
> > and the stated behaviour is to raise there and then.
> > 
> > If a guest enables evtchn while the LAPIC is disabled, then the
> > interrupt is lost.  Like every other interrupt in an x86 system.
> 
> Edge triggered ones you mean, I suppose, but yes.
> 
> > I don't think there is any credible way a guest kernel author can expect
> > the weird evtchn edgecase to wait for an arbitrary point in the future,
> > and it's a corner case that I think is worth not keeping.
> 
> Well - did you look at 7b5b8ca7dffd ("x86/upcall: inject a spurious event
> after setting upcall vector"), referenced by the Fixes: tag? The issue is
> that with evtchn_upcall_pending once set, there would never again be a
> notification. So if what you say is to be the model we follow, then that
> earlier change was perhaps wrong as well. Instead it should then have
> been a guest change (as also implicit from your reply) to clear
> evtchn_upcall_pending after vCPU info registration (there) or LAPIC
> enabling (here), perhaps by way of "manually" invoking the handling of
> that pending event, or by issuing a self-IPI with that vector.
> Especially the LAPIC enabling case would then be yet another Xen-specific
> on a guest code path which better wouldn't have to be aware of Xen.

Another option might be to clear evtchn_upcall_pending once the vLAPIC
is enabled, so that further setting of evtchn_upcall_pending will
inject the vector.  I'm worried however whether that could break
existing users, as this would be an interface behavior change.

Thanks, Roger.