x86/entry/32: Convert IRET exception to IDTENTRY_SW

Convert the IRET exception handler to IDTENTRY_SW. This is slightly
different than the conversions of hardware exceptions as the IRET exception
is invoked via an exception table when IRET faults. So it just uses the
IDTENTRY_SW mechanism for consistency. It does not emit ASM code as it does
not fit the other idtentry exceptions.

  - Implement the C entry point with DEFINE_IDTENTRY_SW() which maps to
    DEFINE_IDTENTRY()
  - Fixup the XEN/PV code
  - Remove the old prototypes
  - Remove the RCU warning as the new entry macro ensures correctness

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505134906.128769226@linutronix.de

arch/x86/include/asm/idtentry.h

@@ -57,6 +57,10 @@ __visible noinstr void func(struct pt_regs *regs)			\
 									\
 static __always_inline void __##func(struct pt_regs *regs)
 
+/* Special case for 32bit IRET 'trap' */
+#define DECLARE_IDTENTRY_SW	DECLARE_IDTENTRY
+#define DEFINE_IDTENTRY_SW	DEFINE_IDTENTRY
+
 /**
  * DECLARE_IDTENTRY_ERRORCODE - Declare functions for simple IDT entry points
  *				Error code pushed by hardware
@@ -111,6 +115,9 @@ static __always_inline void __##func(struct pt_regs *regs,		\
 #define DECLARE_IDTENTRY_ERRORCODE(vector, func)			\
 	idtentry vector asm_##func func has_error_code=1 sane=1
 
+/* Special case for 32bit IRET 'trap'. Do not emit ASM code */
+#define DECLARE_IDTENTRY_SW(vector, func)
+
 #endif /* __ASSEMBLY__ */
 
 /*
@@ -133,6 +140,9 @@ DECLARE_IDTENTRY(X86_TRAP_SPURIOUS,	exc_spurious_interrupt_bug);
 DECLARE_IDTENTRY(X86_TRAP_MF,		exc_coprocessor_error);
 DECLARE_IDTENTRY(X86_TRAP_XF,		exc_simd_coprocessor_error);
 
+/* 32bit software IRET trap. Do not emit ASM code */
+DECLARE_IDTENTRY_SW(X86_TRAP_IRET,	iret_error);
+
 /* Simple exception entries with error code pushed by hardware */
 DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_TS,	exc_invalid_tss);
 DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_NP,	exc_segment_not_present);