x86/headers: Clean up and better document uapi/asm/sigcontext.h

Clean up sigcontext.h:

 - the explanations were full of typos and were hard to read in general
 - use consistent and readable vertical spacing
 - fix, harmonize and extend comments

No field name has been changed, user-space might be relying on
them.

Acked-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1441438363-9999-4-git-send-email-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/uapi/asm/sigcontext.h

@@ -1,221 +1,255 @@
 #ifndef _UAPI_ASM_X86_SIGCONTEXT_H
 #define _UAPI_ASM_X86_SIGCONTEXT_H
 
+/*
+ * Linux signal context definitions. The sigcontext includes a complex hierarchy of CPU
+ * and FPU state, available to user-space (on the stack) when a signal handler is
+ * executed.
+ *
+ * As over the years this ABI grew from its very simple roots towards supporting more and
+ * more CPU state organically, some of the details (which were rather clever hacks back
+ * in the days) became a bit quirky by today.
+ *
+ * The current ABI includes flexible provisions for future extensions, so we won't have
+ * to grow new quirks for quite some time. Promise!
+ */
+
 #include <linux/compiler.h>
 #include <linux/types.h>
 
-#define FP_XSTATE_MAGIC1	0x46505853U
-#define FP_XSTATE_MAGIC2	0x46505845U
-#define FP_XSTATE_MAGIC2_SIZE	sizeof(FP_XSTATE_MAGIC2)
+#define FP_XSTATE_MAGIC1		0x46505853U
+#define FP_XSTATE_MAGIC2		0x46505845U
+#define FP_XSTATE_MAGIC2_SIZE		sizeof(FP_XSTATE_MAGIC2)
 
 /*
- * bytes 464..511 in the current 512byte layout of fxsave/fxrstor frame
- * are reserved for SW usage. On cpu's supporting xsave/xrstor, these bytes
- * are used to extended the fpstate pointer in the sigcontext, which now
+ * Bytes 464..511 in the current 512-byte layout of the FXSAVE/FXRSTOR frame
+ * are reserved for SW usage. On CPUs supporting XSAVE/XRSTOR, these bytes
+ * are used to extend the fpstate pointer in the sigcontext, which now
  * includes the extended state information along with fpstate information.
  *
- * Presence of FP_XSTATE_MAGIC1 at the beginning of this SW reserved
- * area and FP_XSTATE_MAGIC2 at the end of memory layout
- * (extended_size - FP_XSTATE_MAGIC2_SIZE) indicates the presence of the
- * extended state information in the memory layout pointed by the fpstate
- * pointer in sigcontext.
+ * If sw_reserved.magic1 == FP_XSTATE_MAGIC1 then there's a sw_reserved.extended_size
+ * bytes large extended context area present. (The last 32-bit word of this extended
+ * area (at the fpstate+extended_size-FP_XSTATE_MAGIC2_SIZE address) is set to
+ * FP_XSTATE_MAGIC2 so that you can sanity check your size calculations.)
+ *
+ * This extended area typically grows with newer CPUs that have larger and larger
+ * XSAVE areas.
  */
 struct _fpx_sw_bytes {
-	__u32 magic1;		/* FP_XSTATE_MAGIC1 */
-	__u32 extended_size;	/* total size of the layout referred by
-				 * fpstate pointer in the sigcontext.
-				 */
-	__u64 xfeatures;
-				/* feature bit mask (including fp/sse/extended
-				 * state) that is present in the memory
-				 * layout.
-				 */
-	__u32 xstate_size;	/* actual xsave state size, based on the
-				 * features saved in the layout.
-				 * 'extended_size' will be greater than
-				 * 'xstate_size'.
-				 */
-	__u32 padding[7];	/*  for future use. */
+	/* If set to FP_XSTATE_MAGIC1 then this is an xstate context. 0 if a legacy frame. */
+	__u32				magic1;
+
+	/*
+	 * Total size of the fpstate area:
+	 *
+	 *  - if magic1 == 0 then it's sizeof(struct _fpstate)
+	 *  - if magic1 == FP_XSTATE_MAGIC1 then it's sizeof(struct _xstate) plus extensions (if any)
+	 */
+	__u32				extended_size;
+
+	/*
+	 * Feature bit mask (including FP/SSE/extended state) that is present
+	 * in the memory layout:
+	 */
+	__u64				xfeatures;
+
+	/*
+	 * Actual XSAVE state size, based on the xfeatures saved in the layout.
+	 * 'extended_size' is greater than 'xstate_size':
+	 */
+	__u32				xstate_size;
+
+	/* For future use: */
+	__u32				padding[7];
 };
 
 #ifdef __i386__
 /*
- * As documented in the iBCS2 standard..
+ * As documented in the iBCS2 standard:
  *
  * The first part of "struct _fpstate" is just the normal i387
  * hardware setup, the extra "status" word is used to save the
  * coprocessor status word before entering the handler.
  *
- * Pentium III FXSR, SSE support
- *	Gareth Hughes <gareth@valinux.com>, May 2000
- *
  * The FPU state data structure has had to grow to accommodate the
  * extended FPU state required by the Streaming SIMD Extensions.
  * There is no documented standard to accomplish this at the moment.
  */
+
+/* 10-byte legacy floating point register: */
 struct _fpreg {
-	unsigned short significand[4];
-	unsigned short exponent;
+	unsigned short			significand[4];
+	unsigned short			exponent;
 };
 
+/* 16-byte floating point register: */
 struct _fpxreg {
-	unsigned short significand[4];
-	unsigned short exponent;
-	unsigned short padding[3];
+	unsigned short			significand[4];
+	unsigned short			exponent;
+	unsigned short			padding[3];
 };
 
+/* 16-byte XMM register: */
 struct _xmmreg {
-	unsigned long element[4];
+	unsigned long			element[4];
 };
 
+#define X86_FXSR_MAGIC			0x0000
+
 struct _fpstate {
-	/* Regular FPU environment */
-	unsigned long	cw;
-	unsigned long	sw;
-	unsigned long	tag;
-	unsigned long	ipoff;
-	unsigned long	cssel;
-	unsigned long	dataoff;
-	unsigned long	datasel;
-	struct _fpreg	_st[8];
-	unsigned short	status;
-	unsigned short	magic;		/* 0xffff = regular FPU data only */
+	/* Legacy FPU environment: */
+	unsigned long			cw;
+	unsigned long			sw;
+	unsigned long			tag;
+	unsigned long			ipoff;
+	unsigned long			cssel;
+	unsigned long			dataoff;
+	unsigned long			datasel;
+	struct _fpreg			_st[8];
+	unsigned short			status;
+	unsigned short			magic;		/* 0xffff: regular FPU data only */
+							/* 0x0000: FXSR FPU data */
 
 	/* FXSR FPU environment */
-	unsigned long	_fxsr_env[6];	/* FXSR FPU env is ignored */
-	unsigned long	mxcsr;
-	unsigned long	reserved;
-	struct _fpxreg	_fxsr_st[8];	/* FXSR FPU reg data is ignored */
-	struct _xmmreg	_xmm[8];
-	unsigned long	padding1[44];
+	unsigned long			_fxsr_env[6];	/* FXSR FPU env is ignored */
+	unsigned long			mxcsr;
+	unsigned long			reserved;
+	struct _fpxreg			_fxsr_st[8];	/* FXSR FPU reg data is ignored */
+	struct _xmmreg			_xmm[8];	/* First 8 XMM registers */
+	unsigned long			padding1[44];	/* Second 8 XMM registers plus padding */
 
 	union {
-		unsigned long	padding2[12];
-		struct _fpx_sw_bytes sw_reserved; /* represents the extended
-						   * state info */
+		unsigned long		padding2[12];
+		struct _fpx_sw_bytes	sw_reserved;	/* Potential extended state is encoded here */
 	};
 };
 
-#define X86_FXSR_MAGIC		0x0000
-
-#ifndef __KERNEL__
+# ifndef __KERNEL__
 /*
  * User-space might still rely on the old definition:
  */
 struct sigcontext {
-	unsigned short gs, __gsh;
-	unsigned short fs, __fsh;
-	unsigned short es, __esh;
-	unsigned short ds, __dsh;
-	unsigned long edi;
-	unsigned long esi;
-	unsigned long ebp;
-	unsigned long esp;
-	unsigned long ebx;
-	unsigned long edx;
-	unsigned long ecx;
-	unsigned long eax;
-	unsigned long trapno;
-	unsigned long err;
-	unsigned long eip;
-	unsigned short cs, __csh;
-	unsigned long eflags;
-	unsigned long esp_at_signal;
-	unsigned short ss, __ssh;
-	struct _fpstate __user *fpstate;
-	unsigned long oldmask;
-	unsigned long cr2;
+	unsigned short			gs, __gsh;
+	unsigned short			fs, __fsh;
+	unsigned short			es, __esh;
+	unsigned short			ds, __dsh;
+	unsigned long			edi;
+	unsigned long			esi;
+	unsigned long			ebp;
+	unsigned long			esp;
+	unsigned long			ebx;
+	unsigned long			edx;
+	unsigned long			ecx;
+	unsigned long			eax;
+	unsigned long			trapno;
+	unsigned long			err;
+	unsigned long			eip;
+	unsigned short			cs, __csh;
+	unsigned long			eflags;
+	unsigned long			esp_at_signal;
+	unsigned short			ss, __ssh;
+	struct _fpstate __user		*fpstate;
+	unsigned long			oldmask;
+	unsigned long			cr2;
 };
-#endif /* !__KERNEL__ */
+# endif /* !__KERNEL__ */
 
-#else /* __i386__ */
+#else /* __x86_64__: */
 
-/* FXSAVE frame */
-/* Note: reserved1/2 may someday contain valuable data. Always save/restore
-   them when you change signal frames. */
+/*
+ * The FXSAVE frame.
+ *
+ * Note1: If sw_reserved.magic1 == FP_XSTATE_MAGIC1 then the structure is
+ *        larger: 'struct _xstate'. Note that 'struct _xstate' embedds
+ *        'struct _fpstate' so that you can always assume the _fpstate portion
+ *        exists so that you can check the magic value.
+ *
+ * Note2: Reserved fields may someday contain valuable data. Always save/restore
+ *        them when you change signal frames.
+ */
 struct _fpstate {
-	__u16	cwd;
-	__u16	swd;
-	__u16	twd;		/* Note this is not the same as the
-				   32bit/x87/FSAVE twd */
-	__u16	fop;
-	__u64	rip;
-	__u64	rdp;
-	__u32	mxcsr;
-	__u32	mxcsr_mask;
-	__u32	st_space[32];	/* 8*16 bytes for each FP-reg */
-	__u32	xmm_space[64];	/* 16*16 bytes for each XMM-reg  */
-	__u32	reserved2[12];
+	__u16				cwd;
+	__u16				swd;
+	/* Note this is not the same as the 32-bit/x87/FSAVE twd: */
+	__u16				twd;
+	__u16				fop;
+	__u64				rip;
+	__u64				rdp;
+	__u32				mxcsr;
+	__u32				mxcsr_mask;
+	__u32				st_space[32];	/*  8x  FP registers, 16 bytes each */
+	__u32				xmm_space[64];	/* 16x XMM registers, 16 bytes each */
+	__u32				reserved2[12];
 	union {
-		__u32	reserved3[12];
-		struct _fpx_sw_bytes sw_reserved; /* represents the extended
-						   * state information */
+		__u32			reserved3[12];
+		struct _fpx_sw_bytes	sw_reserved;	/* Potential extended state is encoded here */
 	};
 };
 
-#ifndef __KERNEL__
+# ifndef __KERNEL__
 /*
  * User-space might still rely on the old definition:
  */
 struct sigcontext {
-	__u64 r8;
-	__u64 r9;
-	__u64 r10;
-	__u64 r11;
-	__u64 r12;
-	__u64 r13;
-	__u64 r14;
-	__u64 r15;
-	__u64 rdi;
-	__u64 rsi;
-	__u64 rbp;
-	__u64 rbx;
-	__u64 rdx;
-	__u64 rax;
-	__u64 rcx;
-	__u64 rsp;
-	__u64 rip;
-	__u64 eflags;		/* RFLAGS */
-	__u16 cs;
-	__u16 gs;
-	__u16 fs;
-	__u16 __pad0;
-	__u64 err;
-	__u64 trapno;
-	__u64 oldmask;
-	__u64 cr2;
-	struct _fpstate __user *fpstate;	/* zero when no FPU context */
-#ifdef __ILP32__
-	__u32 __fpstate_pad;
-#endif
-	__u64 reserved1[8];
+	__u64				r8;
+	__u64				r9;
+	__u64				r10;
+	__u64				r11;
+	__u64				r12;
+	__u64				r13;
+	__u64				r14;
+	__u64				r15;
+	__u64				rdi;
+	__u64				rsi;
+	__u64				rbp;
+	__u64				rbx;
+	__u64				rdx;
+	__u64				rax;
+	__u64				rcx;
+	__u64				rsp;
+	__u64				rip;
+	__u64				eflags;		/* RFLAGS */
+	__u16				cs;
+	__u16				gs;
+	__u16				fs;
+	__u16				__pad0;
+	__u64				err;
+	__u64				trapno;
+	__u64				oldmask;
+	__u64				cr2;
+	struct _fpstate __user		*fpstate;	/* Zero when no FPU context */
+#  ifdef __ILP32__
+	__u32				__fpstate_pad;
+#  endif
+	__u64				reserved1[8];
 };
-#endif /* !__KERNEL__ */
+# endif /* !__KERNEL__ */
 
-#endif /* !__i386__ */
+#endif /* __x86_64__ */
 
 struct _header {
-	__u64 xfeatures;
-	__u64 reserved1[2];
-	__u64 reserved2[5];
+	__u64				xfeatures;
+	__u64				reserved1[2];
+	__u64				reserved2[5];
 };
 
 struct _ymmh_state {
-	/* 16 * 16 bytes for each YMMH-reg */
-	__u32 ymmh_space[64];
+	/* 16x YMM registers, 16 bytes each: */
+	__u32				ymmh_space[64];
 };
 
 /*
- * Extended state pointed by the fpstate pointer in the sigcontext.
- * In addition to the fpstate, information encoded in the xstate_hdr
- * indicates the presence of other extended state information
- * supported by the processor and OS.
+ * Extended state pointed to by sigcontext::fpstate.
+ *
+ * In addition to the fpstate, information encoded in _xstate::xstate_hdr
+ * indicates the presence of other extended state information supported
+ * by the CPU and kernel:
  */
 struct _xstate {
-	struct _fpstate fpstate;
-	struct _header xstate_hdr;
-	struct _ymmh_state ymmh;
-	/* new processor state extensions go here */
+	struct _fpstate			fpstate;
+	struct _header			xstate_hdr;
+	struct _ymmh_state		ymmh;
+	/* New processor state extensions go here: */
 };
 
 #endif /* _UAPI_ASM_X86_SIGCONTEXT_H */