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@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/b7efc1b7364039824776f68e9ddee9ec1500e894.1426009661.git.luto@amacapital.netSigned-off-by: Ingo Molnar <mingo@kernel.org>

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;