x86/context-tracking: Remove exception_enter/exit() from do_page_fault()

do_page_fault(), like other exceptions, is already covered by
user_enter() and user_exit() when the exception triggers in userspace.

As explained in:

  8c84014 ("x86/entry: Remove exception_enter() from most trap handlers")

exception_enter/exit() only remained to handle possible page fault from
kernel mode while context tracking is in CONTEXT_USER mode, ie: on
kernel entry before we manage to call user_exit(). The only known
offender was do_fast_syscall_32() fetching EBP register from where
vDSO stashed it.

Meanwhile this got fixed in:

  9999c8c ("x86/entry: Call enter_from_user_mode() with IRQs off")

that moved enter_from_user_mode() before the call to get_user().

So we can safely remove it now.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Link: https://lkml.kernel.org/r/20191227163612.10039-2-frederic@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/fault.c

@@ -1486,27 +1486,6 @@ void do_user_addr_fault(struct pt_regs *regs,
 }
 NOKPROBE_SYMBOL(do_user_addr_fault);
 
-/*
- * Explicitly marked noinline such that the function tracer sees this as the
- * page_fault entry point.
- */
-static noinline void
-__do_page_fault(struct pt_regs *regs, unsigned long hw_error_code,
-		unsigned long address)
-{
-	prefetchw(&current->mm->mmap_sem);
-
-	if (unlikely(kmmio_fault(regs, address)))
-		return;
-
-	/* Was the fault on kernel-controlled part of the address space? */
-	if (unlikely(fault_in_kernel_space(address)))
-		do_kern_addr_fault(regs, hw_error_code, address);
-	else
-		do_user_addr_fault(regs, hw_error_code, address);
-}
-NOKPROBE_SYMBOL(__do_page_fault);
-
 static __always_inline void
 trace_page_fault_entries(struct pt_regs *regs, unsigned long error_code,
 			 unsigned long address)
@@ -1521,13 +1500,19 @@ trace_page_fault_entries(struct pt_regs *regs, unsigned long error_code,
 }
 
 dotraplinkage void
-do_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned long address)
+do_page_fault(struct pt_regs *regs, unsigned long hw_error_code,
+		unsigned long address)
 {
-	enum ctx_state prev_state;
+	prefetchw(&current->mm->mmap_sem);
+	trace_page_fault_entries(regs, hw_error_code, address);
 
-	prev_state = exception_enter();
-	trace_page_fault_entries(regs, error_code, address);
-	__do_page_fault(regs, error_code, address);
-	exception_exit(prev_state);
+	if (unlikely(kmmio_fault(regs, address)))
+		return;
+
+	/* Was the fault on kernel-controlled part of the address space? */
+	if (unlikely(fault_in_kernel_space(address)))
+		do_kern_addr_fault(regs, hw_error_code, address);
+	else
+		do_user_addr_fault(regs, hw_error_code, address);
 }
 NOKPROBE_SYMBOL(do_page_fault);