1 files changed, 29 insertions, 0 deletions

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 5f8cfa6b7940..ba243a4cc119 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -617,6 +617,33 @@ handle_io_bitmap(struct thread_struct *next, struct tss_struct *tss)
 }
 
 /*
+ * This function selects if the context switch from prev to next
+ * has to tweak the TSC disable bit in the cr4.
+ */
+static inline void disable_tsc(struct task_struct *prev_p,
+                               struct task_struct *next_p)
+{
+        struct thread_info *prev, *next;
+
+        /*
+         * gcc should eliminate the ->thread_info dereference if
+         * has_secure_computing returns 0 at compile time (SECCOMP=n).
+         */
+        prev = prev_p->thread_info;
+        next = next_p->thread_info;
+
+        if (has_secure_computing(prev) || has_secure_computing(next)) {
+                /* slow path here */
+                if (has_secure_computing(prev) &&
+                    !has_secure_computing(next)) {
+                        write_cr4(read_cr4() & ~X86_CR4_TSD);
+                } else if (!has_secure_computing(prev) &&
+                           has_secure_computing(next))
+                        write_cr4(read_cr4() | X86_CR4_TSD);
+        }
+}
+
+/*
  *      switch_to(x,yn) should switch tasks from x to y.
  *
  * We fsave/fwait so that an exception goes off at the right time
@@ -695,6 +722,8 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
         if (unlikely(prev->io_bitmap_ptr || next->io_bitmap_ptr))
                 handle_io_bitmap(next, tss);
 
+        disable_tsc(prev_p, next_p);
+
         return prev_p;
 }