On Thu, Oct 27, 2022, Paolo Bonzini wrote: > On 10/27/22 16:44, Sean Christopherson wrote: > > > - long mode cannot be changed after the shared info page is enabled (which > > > makes sense because the shared info page also has a compat version) > > > > How is this not introducing an additional restriction? This seems way more > > onerous than what is effectively a revert. > > > > > - the caches must be activated after the shared info page (which enforces > > > that the vCPU attributes are set after the VM attributes) > > > > > > This is technically a userspace API break, but nobody is really using this > > > API outside Amazon so... Patches coming after I finish testing. > > > > It's not just userspace break, it affects the guest ABI as well. > > Yes, I was talking of the VMM here; additional restrictions are fine there. Additional restrictions are fine where? > The guests however should be compatible with Xen, so you also need to > re-activate the cache after the hypercall page is written, but that's two > lines of code. And do what if the guest transitions from 32-bit => 64-bit and the cache isn't aligned for 64-bit? E.g. kvm_xen_set_evtchn() will silently drop events no matter what KVM does. In other words, I don't see how KVM can provide a same ABI without forcing the cached pages to be aligned for the largets possible size.
