x86/entry: Don't call error_entry() for XENPV

XENPV guests enter already on the task stack and they can't fault for
native_iret() nor native_load_gs_index() since they use their own pvop
for IRET and load_gs_index(). A CR3 switch is not needed either.

So there is no reason to call error_entry() in XENPV.

  [ bp: Massage commit message. ]
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220503032107.680190-6-jiangshanlai@gmail.com

arch/x86/entry/entry_64.S

@@ -336,8 +336,17 @@ SYM_CODE_END(push_and_clear_regs)
 	call push_and_clear_regs
 	UNWIND_HINT_REGS
 
-	call	error_entry
-	movq	%rax, %rsp			/* switch to the task stack if from userspace */
+	/*
+	 * Call error_entry() and switch to the task stack if from userspace.
+	 *
+	 * When in XENPV, it is already in the task stack, and it can't fault
+	 * for native_iret() nor native_load_gs_index() since XENPV uses its
+	 * own pvops for IRET and load_gs_index().  And it doesn't need to
+	 * switch the CR3.  So it can skip invoking error_entry().
+	 */
+	ALTERNATIVE "call error_entry; movq %rax, %rsp", \
+		"", X86_FEATURE_XENPV
+
 	ENCODE_FRAME_POINTER
 	UNWIND_HINT_REGS