x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET

Clarify why exactly RF cannot be restored properly by SYSRET to avoid
confusion.

No functionality change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160803171429.GA2590@nazgul.tnic
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9f85827db24e..d172c619c449 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -288,11 +288,15 @@ return_from_SYSCALL_64:
         jne     opportunistic_sysret_failed
 
         /*
-         * SYSRET can't restore RF.  SYSRET can restore TF, but unlike IRET,
-         * restoring TF results in a trap from userspace immediately after
-         * SYSRET.  This would cause an infinite loop whenever #DB happens
-         * with register state that satisfies the opportunistic SYSRET
-         * conditions.  For example, single-stepping this user code:
+         * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
+         * restore RF properly. If the slowpath sets it for whatever reason, we
+         * need to restore it correctly.
+         *
+         * SYSRET can restore TF, but unlike IRET, restoring TF results in a
+         * trap from userspace immediately after SYSRET.  This would cause an
+         * infinite loop whenever #DB happens with register state that satisfies
+         * the opportunistic SYSRET conditions.  For example, single-stepping
+         * this user code:
          *
          *           movq       $stuck_here, %rcx
          *           pushfq