linux-kernel - [PATCH] x86/entry/64: Shorten TEST instructions

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Message-ID: <20171102120926.4srwerqrr7g72e2k@pd.tnic>
Date:   Thu, 2 Nov 2017 13:09:26 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] x86/entry/64: Shorten TEST instructions

On Thu, Nov 02, 2017 at 11:50:18AM +0100, Borislav Petkov wrote:
> If these paths are slow and adding a TEST and a Jcc would give us the
> additional sanity-checking, then I don't see any downside to it.

Damn, that really shows. Almost 900K iterations less.

./lseek1_processes -s 50

before: average:11994233
after : average:11134599

So we'll have to remember to enable CONFIG_DEBUG_ENTRY from time to
time. :-\

Ok, let's then only shorten the TEST insns:

---
From: Borislav Petkov <bp@...e.de>
Date: Thu, 2 Nov 2017 13:00:49 +0100
Subject: [PATCH] x86/entry/64: Shorten TEST instructions

Convert TESTL to TESTB and save 3 bytes per callsite.

No functionality change.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/entry/entry_64.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 40e9933a2d33..84263c79a119 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -620,7 +620,7 @@ GLOBAL(retint_user)
 GLOBAL(swapgs_restore_regs_and_return_to_usermode)
 #ifdef CONFIG_DEBUG_ENTRY
 	/* Assert that pt_regs indicates user mode. */
-	testl	$3, CS(%rsp)
+	testb	$3, CS(%rsp)
 	jnz	1f
 	ud2
 1:
@@ -653,7 +653,7 @@ retint_kernel:
 GLOBAL(restore_regs_and_return_to_kernel)
 #ifdef CONFIG_DEBUG_ENTRY
 	/* Assert that pt_regs indicates kernel mode. */
-	testl	$3, CS(%rsp)
+	testb	$3, CS(%rsp)
 	jz	1f
 	ud2
 1:
-- 
2.13.0

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--