From: Andy Lutomirski <luto@amacapital.net>
To: Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>
Cc: Sebastian Lackner <sebastian@fds-team.de>,
Anish Bhatt <anish@chelsio.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Chuck Ebbert <cebbert.lkml@gmail.com>,
Andy Lutomirski <luto@amacapital.net>
Subject: [PATCH v4 2/2] x86_64: Don't save flags on context switch
Date: Wed, 1 Oct 2014 11:49:05 -0700 [thread overview]
Message-ID: <0102fd968e5b2deffb2bc8771589dc93dee63ead.1412189265.git.luto@amacapital.net> (raw)
In-Reply-To: <cover.1412189265.git.luto@amacapital.net>
In-Reply-To: <cover.1412189265.git.luto@amacapital.net>
Now that the kernel always runs with clean flags (in particular, NT
is clear), there is no need to save and restore flags on every
context switch.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
arch/x86/include/asm/switch_to.h | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index d7f3b3b78ac3..751bf4b7bf11 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -79,12 +79,12 @@ do { \
#else /* CONFIG_X86_32 */
/* frame pointer must be last for get_wchan */
-#define SAVE_CONTEXT "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t"
-#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t"
+#define SAVE_CONTEXT "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
+#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
#define __EXTRA_CLOBBER \
, "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
- "r12", "r13", "r14", "r15"
+ "r12", "r13", "r14", "r15", "flags"
#ifdef CONFIG_CC_STACKPROTECTOR
#define __switch_canary \
@@ -100,7 +100,11 @@ do { \
#define __switch_canary_iparam
#endif /* CC_STACKPROTECTOR */
-/* Save restore flags to clear handle leaking NT */
+/*
+ * There is no need to save or restore flags, because flags are always
+ * clean in kernel mode, with the possible exception of IOPL. Kernel IOPL
+ * has no effect.
+ */
#define switch_to(prev, next, last) \
asm volatile(SAVE_CONTEXT \
"movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */ \
--
1.9.3
next prev parent reply other threads:[~2014-10-01 18:49 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-01 18:49 [PATCH v4 0/2] x86_64,entry: Clear NT on entry and speed up switch_to Andy Lutomirski
2014-10-01 18:49 ` [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace Andy Lutomirski
2014-10-01 19:49 ` Andy Lutomirski
2014-10-02 15:36 ` H. Peter Anvin
2014-10-06 16:42 ` H. Peter Anvin
2014-10-06 18:07 ` [tip:x86/urgent] x86_64, entry: " tip-bot for Andy Lutomirski
2014-10-01 18:49 ` Andy Lutomirski [this message]
2014-10-06 16:39 ` [PATCH v4 0/2] x86_64,entry: Clear NT on entry and speed up switch_to Andy Lutomirski
2014-10-06 16:41 ` H. Peter Anvin
2014-10-06 16:45 ` Andy Lutomirski
2014-10-06 16:57 ` H. Peter Anvin
2014-11-01 0:20 ` Rusty Russell
2014-11-01 1:00 ` Andy Lutomirski
2014-11-01 1:08 ` [PATCH] x86_64, entry: Fix out of bounds read on sysenter Andy Lutomirski
2014-11-01 2:28 ` Rusty Russell
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