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From: Andy Lutomirski <luto@amacapital.net>
To: x86@kernel.org, linux-kernel@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>
Subject: [PATCH 3/3] x86_32: Document our abuse of ss1 and sp1
Date: Tue, 10 Mar 2015 11:06:00 -0700	[thread overview]
Message-ID: <b7efc1b7364039824776f68e9ddee9ec1500e894.1426009661.git.luto@amacapital.net> (raw)
In-Reply-To: <cover.1426009661.git.luto@amacapital.net>
In-Reply-To: <cover.1426009661.git.luto@amacapital.net>

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

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 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 fc6d8d0d8d53..b26208998b7c 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;
-- 
2.3.0


  parent reply	other threads:[~2015-03-10 18:06 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-10 18:05 [PATCH 0/3] sp0, ss1, and sp1 docs and minor fixes Andy Lutomirski
2015-03-10 18:05 ` [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro Andy Lutomirski
2015-03-10 19:22   ` Denys Vlasenko
2015-03-10 19:47     ` Andy Lutomirski
2015-03-13 14:08     ` Denys Vlasenko
2015-03-16  8:56   ` Ingo Molnar
2015-03-16 12:08   ` [tip:x86/asm] x86/asm/entry: Create and use a ' TOP_OF_KERNEL_STACK_PADDING' macro tip-bot for Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski
2015-03-10 18:05 ` [PATCH 2/3] x86: Unify and fix init sp0 Andy Lutomirski
2015-03-11 11:21   ` Borislav Petkov
2015-03-16 12:09   ` [tip:x86/asm] x86/asm/entry: Unify and fix initial thread_struct: :sp0 values tip-bot for Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski
2015-03-10 18:06 ` Andy Lutomirski [this message]
2015-03-10 19:13   ` [PATCH 3/3] x86_32: Document our abuse of ss1 and sp1 Denys Vlasenko
2015-03-10 20:06     ` Andy Lutomirski
2015-03-10 20:52       ` Denys Vlasenko
2015-03-16 12:09   ` [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss: :ss1 and x86_hw_tss::sp1 tip-bot for Andy Lutomirski
2015-03-16 15:36     ` Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski

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