Bug? Incompatible APF for 4.14 guest on 5.10 and later host

["Mancini, Riccardo" <mancio@amazon.com>]

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