Hi, when a 4.14 guest runs on a 5.10 host (and later), it cannot use APF (despite CPUID advertising KVM_FEATURE_ASYNC_PF) due to the new interrupt-based mechanism 2635b5c4a0 (KVM: x86: interrupt based APF 'page ready' event delivery). Kernels after 5.9 won't satisfy the guest request to enable APF through KVM_ASYNC_PF_ENABLED, requiring also KVM_ASYNC_PF_DELIVERY_AS_INT to be set. Furthermore, the patch set seems to be dropping parts of the legacy #PF handling as well. I consider this as a bug as it breaks APF compatibility for older guests running on newer kernels, by breaking the underlying ABI. What do you think? Was this a deliberate decision? Was this already reported in the past (I couldn't find anything in the mailing list but I might have missed it!)? Would it be much effort to support the legacy #PF based mechanism for older guests that choose to only set KVM_ASYNC_PF_ENABLED? The reason this is an issue for us now is that not having APF for older guests introduces a significant performance regression on 4.14 guests when paired to uffd handling of "remote" page-faults (similar to a live migration scenario) when we update from a 4.14 host kernel to a 5.10 host kernel. Thanks, Riccardo
