linux-kernel - [PATCH v2 35/59] x86/entry: Make sync

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Date:   Fri, 02 Sep 2022 15:07:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v2 35/59] x86/entry: Make sync_regs() invocation a tail call

From: Peter Zijlstra <peterz@...radead.org>

No point in having a call there. Spare the call/ret overhead.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/entry/entry_64.S |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1062,11 +1062,8 @@ SYM_CODE_START_LOCAL(error_entry)
 	UNTRAIN_RET
 
 	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
-.Lerror_entry_from_usermode_after_swapgs:
-
 	/* Put us onto the real thread stack. */
-	call	sync_regs
-	RET
+	jmp	sync_regs
 
 	/*
 	 * There are two places in the kernel that can potentially fault with
@@ -1124,7 +1121,7 @@ SYM_CODE_START_LOCAL(error_entry)
 	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
 	call	fixup_bad_iret
 	mov	%rax, %rdi
-	jmp	.Lerror_entry_from_usermode_after_swapgs
+	jmp	sync_regs
 SYM_CODE_END(error_entry)
 
 SYM_CODE_START_LOCAL(error_return)