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X-MS-Exchange-CrossTenant-AuthSource: SJ0PR03MB6360.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Nov 2022 13:55:35.5640 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: K+6SxyH+CNqvEdaANgOFQLe8Y5e1yEIa4Gz4vIYYHprN3XTlmoroE/sNad7aSPKYBOYg7EF25phm5W+g6LyFcA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN4PR03MB6752 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.