linux-kernel - [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss: :ss1 and x86_hw

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Message-ID: <tip-f7113ffa3bcd630066119723f539db75a5721c88@git.kernel.org>
Date:	Mon, 16 Mar 2015 05:09:30 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	oleg@...hat.com, mingo@...nel.org, bp@...en8.de,
	luto@...capital.net, dvlasenk@...hat.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, hpa@...or.com
Subject: [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss:
 :ss1 and x86_hw_tss::sp1

Commit-ID:  f7113ffa3bcd630066119723f539db75a5721c88
Gitweb:     http://git.kernel.org/tip/f7113ffa3bcd630066119723f539db75a5721c88
Author:     Andy Lutomirski <luto@...capital.net>
AuthorDate: Tue, 10 Mar 2015 11:06:00 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 16 Mar 2015 11:05:36 +0100

x86/asm/entry/32: Document our abuse of x86_hw_tss::ss1 and x86_hw_tss::sp1

This has confused me for a while.  Now that I figured it out, document it.

Signed-off-by: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/b7efc1b7364039824776f68e9ddee9ec1500e894.1426009661.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/include/asm/processor.h | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index aed6d4f..c3a037b 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -209,9 +209,24 @@ struct x86_hw_tss {
 	unsigned short		back_link, __blh;
 	unsigned long		sp0;
 	unsigned short		ss0, __ss0h;
-	unsigned long		sp1;
-	/* ss1 caches MSR_IA32_SYSENTER_CS: */
-	unsigned short		ss1, __ss1h;
+
+	/*
+	 * We don't use ring 1, so sp1 and ss1 are convenient scratch
+	 * spaces in the same cacheline as sp0.  We use them to cache
+	 * some MSR values to avoid unnecessary wrmsr instructions.
+	 *
+	 * We use SYSENTER_ESP to find sp0 and for the NMI emergency
+	 * stack, but we need to context switch it because we do
+	 * horrible things to the kernel stack in vm86 mode.
+	 *
+	 * We use SYSENTER_CS to disable sysenter in vm86 mode to avoid
+	 * corrupting the stack if we went through the sysenter path
+	 * from vm86 mode.
+	 */
+	unsigned long		sp1;	/* MSR_IA32_SYSENTER_ESP */
+	unsigned short		ss1;	/* MSR_IA32_SYSENTER_CS */
+
+	unsigned short		__ss1h;
 	unsigned long		sp2;
 	unsigned short		ss2, __ss2h;
 	unsigned long		__cr3;
--
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