x86/fpu: Disable MPX when eagerfpu is off

This issue is a fallout from the command-line parsing move.

When "eagerfpu=off" is given as a command-line input, the kernel
should disable MPX support. The decision for turning off MPX was
made in fpu__init_system_ctx_switch(), which is after the
selection of the XSAVE format. This patch fixes it by getting
that decision done earlier in fpu__init_system_xstate().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: yu-cheng yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/1452119094-7252-4-git-send-email-yu-cheng.yu@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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

@@ -42,6 +42,7 @@ extern void fpu__init_cpu_xstate(void);
 extern void fpu__init_system(struct cpuinfo_x86 *c);
 extern void fpu__init_check_bugs(void);
 extern void fpu__resume_cpu(void);
+extern u64 fpu__get_supported_xfeatures_mask(void);
 
 /*
  * Debugging facility:

arch/x86/kernel/fpu/init.c

@@ -273,8 +273,46 @@ static void __init fpu__init_system_xstate_size_legacy(void)
  */
 static enum { AUTO, ENABLE, DISABLE } eagerfpu = AUTO;
 
+/*
+ * Find supported xfeatures based on cpu features and command-line input.
+ * This must be called after fpu__init_parse_early_param() is called and
+ * xfeatures_mask is enumerated.
+ */
+u64 __init fpu__get_supported_xfeatures_mask(void)
+{
+	/* Support all xfeatures known to us */
+	if (eagerfpu != DISABLE)
+		return XCNTXT_MASK;
+
+	/* Warning of xfeatures being disabled for no eagerfpu mode */
+	if (xfeatures_mask & XFEATURE_MASK_EAGER) {
+		pr_err("x86/fpu: eagerfpu switching disabled, disabling the following xstate features: 0x%llx.\n",
+			xfeatures_mask & XFEATURE_MASK_EAGER);
+	}
+
+	/* Return a mask that masks out all features requiring eagerfpu mode */
+	return ~XFEATURE_MASK_EAGER;
+}
+
+/*
+ * Disable features dependent on eagerfpu.
+ */
+static void __init fpu__clear_eager_fpu_features(void)
+{
+	setup_clear_cpu_cap(X86_FEATURE_MPX);
+}
+
 /*
  * Pick the FPU context switching strategy:
+ *
+ * When eagerfpu is AUTO or ENABLE, we ensure it is ENABLE if either of
+ * the following is true:
+ *
+ * (1) the cpu has xsaveopt, as it has the optimization and doing eager
+ *     FPU switching has a relatively low cost compared to a plain xsave;
+ * (2) the cpu has xsave features (e.g. MPX) that depend on eager FPU
+ *     switching. Should the kernel boot with noxsaveopt, we support MPX
+ *     with eager FPU switching at a higher cost.
  */
 static void __init fpu__init_system_ctx_switch(void)
 {
@@ -286,19 +324,11 @@ static void __init fpu__init_system_ctx_switch(void)
 	WARN_ON_FPU(current->thread.fpu.fpstate_active);
 	current_thread_info()->status = 0;
 
-	/* Auto enable eagerfpu for xsaveopt */
 	if (boot_cpu_has(X86_FEATURE_XSAVEOPT) && eagerfpu != DISABLE)
 		eagerfpu = ENABLE;
 
-	if (xfeatures_mask & XFEATURE_MASK_EAGER) {
-		if (eagerfpu == DISABLE) {
-			pr_err("x86/fpu: eagerfpu switching disabled, disabling the following xstate features: 0x%llx.\n",
-			       xfeatures_mask & XFEATURE_MASK_EAGER);
-			xfeatures_mask &= ~XFEATURE_MASK_EAGER;
-		} else {
-			eagerfpu = ENABLE;
-		}
-	}
+	if (xfeatures_mask & XFEATURE_MASK_EAGER)
+		eagerfpu = ENABLE;
 
 	if (eagerfpu == ENABLE)
 		setup_force_cpu_cap(X86_FEATURE_EAGER_FPU);
@@ -316,10 +346,12 @@ static void __init fpu__init_parse_early_param(void)
 	 * No need to check "eagerfpu=auto" again, since it is the
 	 * initial default.
 	 */
-	if (cmdline_find_option_bool(boot_command_line, "eagerfpu=off"))
+	if (cmdline_find_option_bool(boot_command_line, "eagerfpu=off")) {
 		eagerfpu = DISABLE;
-	else if (cmdline_find_option_bool(boot_command_line, "eagerfpu=on"))
+		fpu__clear_eager_fpu_features();
+	} else if (cmdline_find_option_bool(boot_command_line, "eagerfpu=on")) {
 		eagerfpu = ENABLE;
+	}
 
 	if (cmdline_find_option_bool(boot_command_line, "no387"))
 		setup_clear_cpu_cap(X86_FEATURE_FPU);

arch/x86/kernel/fpu/xstate.c

@@ -633,8 +633,7 @@ void __init fpu__init_system_xstate(void)
 		BUG();
 	}
 
-	/* Support only the state known to the OS: */
-	xfeatures_mask = xfeatures_mask & XCNTXT_MASK;
+	xfeatures_mask &= fpu__get_supported_xfeatures_mask();
 
 	/* Enable xstate instructions to be able to continue with initialization: */
 	fpu__init_cpu_xstate();