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

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index d0ce5cf..ab7b3a2 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -277,6 +277,7 @@
 #define X86_FEATURE_XSAVEC             (10*32+ 1) /* XSAVEC instruction */
 #define X86_FEATURE_XGETBV1            (10*32+ 2) /* XGETBV with ECX = 1 instruction */
 #define X86_FEATURE_XSAVES             (10*32+ 3) /* XSAVES/XRSTORS instructions */
+#define X86_FEATURE_XFD                        (10*32+ 4) /* "" eXtended Feature Disabling */
 
 /*
  * Extended auxiliary flags: Linux defined - for features scattered in various

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index defda61..d9ead9c 100644 (file)
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -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    },
        {}
 };