x86/fpu: Add fpu_state_config::legacy_features

The upcoming prctl() which is required to request the permission for a
dynamically enabled feature will also provide an option to retrieve the
supported features. If the CPU does not support XSAVE, the supported
features would be 0 even when the CPU supports FP and SSE.

Provide separate storage for the legacy feature set to avoid that and fill
in the bits in the legacy init function.

Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Chang S. Bae <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/include/asm/fpu/types.h

···
 	 * be requested by user space before usage.
 	 */
 	u64 default_features;
+	/*
+	 * @legacy_features:
+	 *
+	 * Features which can be reported back to user space
+	 * even without XSAVE support, i.e. legacy features FP + SSE
+	 */
+	u64 legacy_features;
 };
 
 /* FPU state configuration information */

arch/x86/kernel/fpu/init.c

···
 	 * Note that the size configuration might be overwritten later
 	 * during fpu__init_system_xstate().
 	 */
-	if (!cpu_feature_enabled(X86_FEATURE_FPU))
+	if (!cpu_feature_enabled(X86_FEATURE_FPU)) {
 		size = sizeof(struct swregs_state);
-	else if (cpu_feature_enabled(X86_FEATURE_FXSR))
+	} else if (cpu_feature_enabled(X86_FEATURE_FXSR)) {
 		size = sizeof(struct fxregs_state);
-	else
+		fpu_user_cfg.legacy_features = XFEATURE_MASK_FPSSE;
+	} else {
 		size = sizeof(struct fregs_state);
+		fpu_user_cfg.legacy_features = XFEATURE_MASK_FP;
+	}
 
 	fpu_kernel_cfg.max_size = size;
 	fpu_kernel_cfg.default_size = size;