[tip:x86/urgent] x86/entry/64: Further improve paranoid_entry comments

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  0cd6fa95a4a44ff2d649957423946c19eb98e825
Gitweb:     https://git.kernel.org/tip/0cd6fa95a4a44ff2d649957423946c19eb98e825
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Sun, 14 Oct 2018 11:38:18 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 16 Oct 2018 08:19:34 +0200

x86/entry/64: Further improve paranoid_entry comments

Commit:

  16561f27f94e ("x86/entry: Add some paranoid entry/exit CR3 handling comments")

... added some comments.  This improves them a bit:

 - When I first read the new comments, it was unclear to me whether
   they were referring to the case where paranoid_entry interrupted
   other entry code or where paranoid_entry was itself interrupted.
   Clarify it.

 - Remove the EBX comment.  We no longer use EBX as a SWAPGS
   indicator.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/c47daa1888dc2298e7e1d3f82bd76b776ea33393.1539542111.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/entry/entry_64.S | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1d9b4a300c8c..f95dcb209fdf 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1189,15 +1189,13 @@ ENTRY(paranoid_entry)
 1:
 	/*
 	 * Always stash CR3 in %r14.  This value will be restored,
-	 * verbatim, at exit.  Needed if kernel is interrupted
-	 * after switching to the user CR3 value but before
-	 * returning to userspace.
+	 * verbatim, at exit.  Needed if paranoid_entry interrupted
+	 * another entry that already switched to the user CR3 value
+	 * but has not yet returned to userspace.
 	 *
 	 * This is also why CS (stashed in the "iret frame" by the
 	 * hardware at entry) can not be used: this may be a return
-	 * to kernel code, but with a user CR3 value.  The %ebx flag
-	 * for SWAPGS is also unusable for CR3 because there is a
-	 * window with a user GS and a kernel CR3.
+	 * to kernel code, but with a user CR3 value.
 	 */
 	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14