linux-kernel - [PATCH V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error

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Date:   Thu, 21 Apr 2022 22:10:50 +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>,
        Juergen Gross <jgross@...e.com>, 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 V6 3/8] x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()

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

The macro idtentry calls error_entry() unconditionally even on XENPV.
But the code XENPV needs in error_entry() is PUSH_AND_CLEAR_REGS only.
And error_entry() also calls sync_regs() which has to deal with the
case of XENPV via an extra branch so that it doesn't copy the pt_regs.

And PUSH_AND_CLEAR_REGS in error_entry() makes the stack not return to
its original place when the function returns, which means it is not
possible to convert it to a C function.

Move PUSH_AND_CLEAR_REGS out of error_entry(), add a function to wrap
PUSH_AND_CLEAR_REGS and call it before error_entry().

The new function call adds two instructions (CALL and RET) for every
interrupt or exception.  It will allow for error_entry() to be not
called on XENPV which allows for sync_regs() to reduce a branch.  It
will also allow for error_entry() to be converted to C code that can
use inlined sync_regs() and save a function call again.

Cc: Juergen Gross <jgross@...e.com>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>
---
 arch/x86/entry/entry_64.S | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index ca3e99e08a44..b1cef3b0a7ab 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -318,6 +318,14 @@ SYM_CODE_END(ret_from_fork)
 #endif
 .endm

+/* Save all registers in pt_regs */
+SYM_CODE_START_LOCAL(push_and_clear_regs)
+	UNWIND_HINT_FUNC
+	PUSH_AND_CLEAR_REGS save_ret=1
+	ENCODE_FRAME_POINTER 8
+	RET
+SYM_CODE_END(push_and_clear_regs)
+
 /**
  * idtentry_body - Macro to emit code calling the C function
  * @cfunc:		C function to be called
@@ -325,6 +333,9 @@ SYM_CODE_END(ret_from_fork)
  */
 .macro idtentry_body cfunc has_error_code:req

+	call push_and_clear_regs
+	UNWIND_HINT_REGS
+
 	call	error_entry
 	movq	%rax, %rsp			/* switch to the task stack if from userspace */
 	ENCODE_FRAME_POINTER
@@ -985,13 +996,11 @@ SYM_CODE_START_LOCAL(paranoid_exit)
 SYM_CODE_END(paranoid_exit)

 /*
- * Save all registers in pt_regs, and switch GS if needed.
+ * Switch GS and CR3 if needed.
  */
 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