Documentation/x86: Explain the state component permission for guests

Commit 980fe2fd ("x86/fpu: Extend fpu_xstate_prctl() with guest
permissions") extends a couple of arch_prctl(2) options for VCPU threads.
Add description for them.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Yang Zhong <yang.zhong@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/all/20230121001900.14900-5-chang.seok.bae%40intel.com

Documentation/x86/xstate.rst

@@ -143,3 +143,32 @@ entry if the feature is in its initial configuration.  This differs from
 non-dynamic features which are always written regardless of their
 configuration.  Signal handlers can examine the XSAVE buffer's XSTATE_BV
 field to determine if a features was written.
+
+Dynamic features for virtual machines
+-------------------------------------
+
+The permission for the guest state component needs to be managed separately
+from the host, as they are exclusive to each other. A coupled of options
+are extended to control the guest permission:
+
+-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 the guest
+ components.
+
+-ARCH_REQ_XCOMP_GUEST_PERM
+
+ arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM, feature_nr);
+
+ ARCH_REQ_XCOMP_GUEST_PERM is a variant of ARCH_REQ_XCOMP_PERM. It has the
+ same semantics for the guest permission. While providing a similar
+ functionality, this comes with a constraint. Permission is frozen when the
+ first VCPU is created. Any attempt to change permission after that point
+ is going to be rejected. So, the permission has to be requested before the
+ first VCPU creation.
+
+Note that some VMMs may have already established a set of supported state
+components. These options are not presumed to support any particular VMM.