On Wed, 9 Sep 2020 at 16:48, Andrew Jones <drjo...@redhat.com> wrote: > We either need a KVM cap or a new CPU feature probing interface to avoid > making userspace try features one at a time. It's too bad that VCPU_INIT > doesn't clear all offending features from the feature set when returning > EINVAL, because then userspace could create a scratch VCPU with everything > it supports in order to see what KVM also supports in one go.
You could add one if you wanted -- add a new feature bit TELL_ME_WHAT_YOU_HAVE. If the kernel sees that then on filure it clears out feature bits it doesn't support and also clears TELL_ME_WHAT_YOU_HAVE. If QEMU sees EINVAL and TELL_ME_WHAT_YOU_HAVE is still set, then it knows it's dealing with an old kernel and has to do one-at-a-time probing. If it sees EINVAL but not TELL_ME_WHAT_YOU_HAVE then it knows it has a new kernel and has just got all the info. -- PMM
