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Bae wrote: > On 6/16/2022 3:49 PM, Dave Hansen wrote: >> >> This touches on the "what", but not the "why".  Could you explain in >> here both why this is needed and why an app might want to use it? > > [ while studying on this a bit further, found a few things here ] > > They (ARCH_{REQ|GET}_XCOMP_GUEST_PERM) provide a userspace VMM to > request & check guest permission. > > In general, KVM looks to have an API as a set of ioctls [1]. A guest VMM > uses KVM_GET_DEVICE_ATTR::KVM_X86_XCOMP_GUEST_SUPP to query the > available features [2][3]. ARCH_GET_XCOMP_SUPP is not usable here > because KVM wants to control those exposed features [4] (via > KVM_SUPPORTED_XCR0). > > But oddly this mask does not appear to be actively referenced by those > two arch_prctl options. I can see this ioctl attribute is currently > disconnected from these arch_prctl options. > > Also I failed to find the documentation about this > KVM_X86_XCOMP_GUEST_SUPP interface: > >     $ git grep KVM_X86_XCOMP_GUEST_SUPP ./Documentation/ >     $ > > I guess people will be confused with having these two options only. I > think documenting this has to come along with these missing pieces (and > potential fix). So I'm inclined to drop this one at the moment. Posted this series as following up this: https://lore.kernel.org/lkml/20220823231402.7839-1-chang.seok.bae@intel.com/ Thanks, Chang