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

From: tip-bot for Andy Lutomirski
Date: Tue Oct 16 2018 - 02:23:44 EST

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

x86/entry/64: Further improve paranoid_entry comments


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

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/c47daa1888dc2298e7e1d3f82bd76b776ea33393.1539542111.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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)
* 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