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Documentation/x86: Explain the state component permission for guests

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Commit 980fe2fddc ("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
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Chang S. Bae 2023-01-20 16:19:00 -08:00 committed by Dave Hansen
parent 7f9daaf59e
commit 5fbff26075
1 changed files with 29 additions and 0 deletions
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Documentation/x86/xstate.rst
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@ -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.