[tip: x86/iopl] x86/entry/32: Clarify register saving in __switch_to_asm()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/iopl branch of tip:

Commit-ID:     a3ba966066afbe8fd0d3605ffe04c633083752f1
Gitweb:        https://git.kernel.org/tip/a3ba966066afbe8fd0d3605ffe04c633083752f1
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Sat, 16 Nov 2019 11:12:03 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sat, 16 Nov 2019 11:24:07 +01:00

x86/entry/32: Clarify register saving in __switch_to_asm()

commit 6690e86be83a ("sched/x86: Save [ER]FLAGS on context switch")
re-introduced the flags saving on context switch to prevent AC leakage.

The pushf/popf instructions are right among the callee saved register
section, so the comment explaining the save/restore is not entirely
correct.

Add a seperate comment to pushf/popf explaining the reason.

Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/entry/entry_32.S | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index f83ca5a..99fad6f 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -718,6 +718,11 @@ ENTRY(__switch_to_asm)
 	pushl	%ebx
 	pushl	%edi
 	pushl	%esi
+	/*
+	 * Flags are saved to prevent AC leakage. This could go
+	 * away if objtool would have 32bit support to verify
+	 * the STAC/CLAC correctness.
+	 */
 	pushfl
 
 	/* switch stack */
@@ -740,8 +745,9 @@ ENTRY(__switch_to_asm)
 	FILL_RETURN_BUFFER %ebx, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
 #endif
 
-	/* restore callee-saved registers */
+	/* Restore flags or the incoming task to restore AC state. */
 	popfl
+	/* restore callee-saved registers */
 	popl	%esi
 	popl	%edi
 	popl	%ebx