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.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index fc6d8d0d8d5359043b8feeb4b874fece37db131f..b26208998b7cbd7462f4784f1a20c9eba3aa9344 100644 (file)
--- 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;