[PATCH V2 5/7] x86/entry: Don't call error_entry for XENPV

[Lai Jiangshan <jiangshanlai@xxxxxxxxx>]

From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>

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 CR3.
So there is no reason to call error_entry() in XENPV.

Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 630bf8164a09..adc9f7619d1b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -325,8 +325,17 @@ SYM_CODE_END(ret_from_fork)
        PUSH_AND_CLEAR_REGS
        ENCODE_FRAME_POINTER
 
-       call    error_entry
-       movq    %rax, %rsp                      /* switch stack settled by 
sync_regs() */
+       /*
+        * Call error_entry and switch stack settled by sync_regs().
+        *
+        * 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 CR3.  So it can skip invoking error_entry().
+        */
+       ALTERNATIVE "call error_entry; movq %rax, %rsp", \
+               "", X86_FEATURE_XENPV
+
        ENCODE_FRAME_POINTER
        UNWIND_HINT_REGS
 
-- 
2.19.1.6.gb485710b