On Wed, 9 Sep 2020 at 16:48, Andrew Jones [off-list ref] 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

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