linux-kernel - [PATCH V5 3/7] x86/entry: Move PUSH_AND_CLEAR_REGS out of error

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Date:   Tue, 12 Apr 2022 20:15:37 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH V5 3/7] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()

From: Lai Jiangshan <jiangshan.ljs@...group.com>

error_entry() doesn't handle the stack balanced.  It includes
PUSH_AND_CLEAR_REGS which is commonly needed for all IDT entries and
can't pop the regs before it returns.

Move PUSH_AND_CLEAR_REGS out of error_entry() and make error_entry()
works on the stack normally.

After this, XENPV doesn't need error_entry() since PUSH_AND_CLEAR_REGS
is moved out and error_entry() can be converted to C code in future
since it doesn't fiddle the stack.

The text size will be enlarged:

size arch/x86/entry/entry_64.o.before:
   text	   data	    bss	    dec	    hex	filename
  17916	    384	      0	  18300	   477c	arch/x86/entry/entry_64.o

size --format=SysV arch/x86/entry/entry_64.o.before:
.entry.text                      5528      0
.orc_unwind                      6456      0
.orc_unwind_ip                   4304      0

size arch/x86/entry/entry_64.o.after:
   text	   data	    bss	    dec	    hex	filename
  26868	    384	      0	  27252	   6a74	arch/x86/entry/entry_64.o

size --format=SysV arch/x86/entry/entry_64.o.after:
.entry.text                      8200      0
.orc_unwind                     10224      0
.orc_unwind_ip                   6816      0

The tables .orc_unwind[_ip] are enlarged due to it adds many pushes.

But .entry.text in x86_64 is 2M aligned, enlarging it to 8.2k doesn't
enlarge the final text size.

And it will only increase the footprint when different interrupts and
exceptions happen unlikely heavily at the same time.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>
---
 arch/x86/entry/entry_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e1efc56fbcd4..835b798556fb 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -325,6 +325,9 @@ SYM_CODE_END(ret_from_fork)
  */
 .macro idtentry_body cfunc has_error_code:req
 
+	PUSH_AND_CLEAR_REGS
+	ENCODE_FRAME_POINTER
+
 	call	error_entry
 	movq	%rax, %rsp			/* switch to the task stack if from userspace */
 	ENCODE_FRAME_POINTER
@@ -987,8 +990,6 @@ SYM_CODE_END(paranoid_exit)
 SYM_CODE_START_LOCAL(error_entry)
 	UNWIND_HINT_FUNC
 	cld
-	PUSH_AND_CLEAR_REGS save_ret=1
-	ENCODE_FRAME_POINTER 8
 	testb	$3, CS+8(%rsp)
 	jz	.Lerror_kernelspace
 
-- 
2.19.1.6.gb485710b