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>

3 files changed, 46 insertions, 14 deletions

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index eadcdd5bb946..0fd440df63f1 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/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:
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 3a45fcd0b924..f0ab36844a6d 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -274,7 +274,45 @@ 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);
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index d489f277a8a0..d425cda5ae6d 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/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();