Commit c3511016 authored Oct 21, 2021 by Chang S. Bae Committed by Borislav Petkov Oct 26, 2021

x86/cpufeatures: Add eXtended Feature Disabling (XFD) feature bit



Intel's eXtended Feature Disable (XFD) feature is an extension of the XSAVE
architecture. XFD allows the kernel to enable a feature state in XCR0 and
to receive a #NM trap when a task uses instructions accessing that state.

This is going to be used to postpone the allocation of a larger XSTATE
buffer for a task to the point where it is actually using a related
instruction after the permission to use that facility has been granted.

XFD is not used by the kernel, but only applied to userspace. This is a
matter of policy as the kernel knows how a fpstate is reallocated and the
XFD state.

The compacted XSAVE format is adjustable for dynamic features. Make XFD
depend on XSAVES.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021225527.10184-13-chang.seok.bae@intel.com

parent e61d6310

arch/x86/include/asm/cpufeatures.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -277,6 +277,7 @@
		#define X86_FEATURE_XSAVEC (1032+ 1) / XSAVEC instruction */
		#define X86_FEATURE_XGETBV1 (1032+ 2) / XGETBV with ECX = 1 instruction */
		#define X86_FEATURE_XSAVES (1032+ 3) / XSAVES/XRSTORS instructions */
		#define X86_FEATURE_XFD (1032+ 4) / "" eXtended Feature Disabling */

		/*
		* Extended auxiliary flags: Linux defined - for features scattered in various

arch/x86/kernel/cpu/cpuid-deps.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -75,6 +75,7 @@ static const struct cpuid_dep cpuid_deps[] = {
		{ X86_FEATURE_SGX_LC, X86_FEATURE_SGX },
		{ X86_FEATURE_SGX1, X86_FEATURE_SGX },
		{ X86_FEATURE_SGX2, X86_FEATURE_SGX1 },
		{ X86_FEATURE_XFD, X86_FEATURE_XSAVES },
		{}
		};