Messages in this thread | | | Date | Thu, 23 Jun 2022 16:55:41 -0700 | Subject | Re: [PATCH 2/2] Documentation/x86: Explain guest XSTATE permission control | From | "Chang S. Bae" <> |
| |
On 6/16/2022 3:49 PM, Dave Hansen wrote: > On 6/16/22 14:22, Chang S. Bae wrote: >> +In addition, a couple of extended options are provided for a VCPU thread. >> +The VCPU XSTATE permission is separately controlled. >> + >> +-ARCH_GET_XCOMP_GUEST_PERM >> + >> + arch_prctl(ARCH_GET_XCOMP_GUEST_PERM, &features); >> + >> + ARCH_GET_XCOMP_GUEST_PERM is a variant of ARCH_GET_XCOMP_PERM. So it >> + provides the same semantics and functionality but for VCPU. > > 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.
Thanks, Chang
[1] https://kernel.org/doc/html/latest/virt/kvm/index.html [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/kvm/lib/x86_64/processor.c#n641 [3] https://github.com/qemu/qemu/blob/58b53669e87fed0d70903e05cd42079fbbdbc195/target/i386/kvm/kvm.c#L428 [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kvm/x86.c#n9008
| |