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From: Andy Lutomirski <luto@kernel.org>
To: x86@kernel.org
Cc: linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
	Oleg Nesterov <oleg@redhat.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Brian Gerst <brgerst@gmail.com>,
	Andy Lutomirski <luto@kernel.org>
Subject: [PATCH v2 03/12] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER
Date: Wed,  9 Mar 2016 19:00:26 -0800	[thread overview]
Message-ID: <07daef3d44bd1ed62a2c866e143e8df64edb40ee.1457578375.git.luto@kernel.org> (raw)
In-Reply-To: <cover.1457578375.git.luto@kernel.org>
In-Reply-To: <cover.1457578375.git.luto@kernel.org>

This makes the 32-bit code work just like the 64-bit code.  It should
speed up syscalls on 32-bit kernels on Skylake by something like 20
cycles (by analogy to the 64-bit compat case).

It also cleans up NT just like we do for the 64-bit case.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_32.S | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 684d2505b93a..86665f93ff5d 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -294,7 +294,6 @@ sysenter_past_esp:
 	pushl	$__USER_DS		/* pt_regs->ss */
 	pushl	%ebp			/* pt_regs->sp (stashed in bp) */
 	pushfl				/* pt_regs->flags (except IF = 0) */
-	ASM_CLAC			/* Clear AC after saving FLAGS */
 	orl	$X86_EFLAGS_IF, (%esp)	/* Fix IF */
 	pushl	$__USER_CS		/* pt_regs->cs */
 	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
@@ -302,6 +301,23 @@ sysenter_past_esp:
 	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest */
 
 	/*
+	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
+	 * ourselves.  To save a few cycles, we can check whether
+	 * either was set instead of doing an unconditional popfq.
+	 * This needs to happen before enabling interrupts so that
+	 * we don't get preempted with NT set.
+	 *
+	 * NB.: .Lsysenter_fix_flags is a label with the code under it moved
+	 * out-of-line as an optimization: NT is unlikely to be set in the
+	 * majority of the cases and instead of polluting the I$ unnecessarily,
+	 * we're keeping that code behind a branch which will predict as
+	 * not-taken and therefore its instructions won't be fetched.
+	 */
+	testl	$X86_EFLAGS_NT|X86_EFLAGS_AC, PT_EFLAGS(%esp)
+	jnz	.Lsysenter_fix_flags
+.Lsysenter_flags_fixed:
+
+	/*
 	 * User mode is traced as though IRQs are on, and SYSENTER
 	 * turned them off.
 	 */
@@ -339,6 +355,11 @@ sysenter_past_esp:
 .popsection
 	_ASM_EXTABLE(1b, 2b)
 	PTGS_TO_GS_EX
+
+.Lsysenter_fix_flags:
+	pushl	$X86_EFLAGS_FIXED
+	popfl
+	jmp	.Lsysenter_flags_fixed
 ENDPROC(entry_SYSENTER_32)
 
 	# system call handler stub
-- 
2.5.0

  parent reply	other threads:[~2016-03-10  3:01 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-10  3:00 [PATCH v2 00/12] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 01/12] selftests/x86: In syscall_nt, test NT|TF as well Andy Lutomirski
2016-03-10 11:00   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 02/12] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test Andy Lutomirski
2016-03-10 11:01   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` Andy Lutomirski [this message]
2016-03-10 11:01   ` [tip:x86/asm] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 04/12] x86/entry/32: Restore FLAGS on SYSEXIT Andy Lutomirski
2016-03-10 11:01   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 05/12] x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions Andy Lutomirski
2016-03-10 11:02   ` [tip:x86/asm] x86/entry/traps: " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 06/12] x86/traps: Clear DR6 early in do_debug and improve the comment Andy Lutomirski
2016-03-10 11:02   ` [tip:x86/asm] x86/entry/traps: Clear DR6 early in do_debug() " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 07/12] x86/entry: Vastly simplify SYSENTER TF handling Andy Lutomirski
2016-03-10 11:03   ` [tip:x86/asm] x86/entry: Vastly simplify SYSENTER TF (single-step) handling tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 08/12] x86/entry: Only allocate space for SYSENTER_stack if needed Andy Lutomirski
2016-03-10 11:03   ` [tip:x86/asm] x86/entry: Only allocate space for tss_struct::SYSENTER_stack " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 09/12] x86/entry/32: Simplify and fix up the SYSENTER stack #DB/NMI fixup Andy Lutomirski
2016-03-10 11:03   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 10/12] x86/entry/32: Add and check a stack canary for the SYSENTER stack Andy Lutomirski
2016-03-10 11:04   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 11/12] x86/entry: Remove TIF_SINGLESTEP entry work Andy Lutomirski
2016-03-10 11:04   ` [tip:x86/asm] " tip-bot for Andy Lutomirski
2016-03-10  3:00 ` [PATCH v2 12/12] x86/entry: Improve system call entry comments Andy Lutomirski
2016-03-10 11:05   ` [tip:x86/asm] " tip-bot for Andy Lutomirski

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