From: Chang S. Bae Date: Sat, 21 Jan 2023 00:19:00 +0000 (-0800) Subject: Documentation/x86: Explain the state component permission for guests X-Git-Tag: v6.6.7~2966^2 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=5fbff260755750559aa12a30f6fa7f8a863666f1;p=platform%2Fkernel%2Flinux-starfive.git Documentation/x86: Explain the state component permission for guests Commit 980fe2fddcff ("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 Signed-off-by: Dave Hansen Reviewed-by: Thiago Macieira Reviewed-by: Yang Zhong Reviewed-by: Tony Luck Link: https://lore.kernel.org/all/20230121001900.14900-5-chang.seok.bae%40intel.com --- diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst index 23b1c9f..ae5c69e 100644 --- a/Documentation/x86/xstate.rst +++ b/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.