linux-kernel - [PATCH 11/11] x86/entry: Remove the branch in sync

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Date:   Wed,  8 Dec 2021 19:08:33 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>
Subject: [PATCH 11/11] x86/entry: Remove the branch in sync_regs()

From: Lai Jiangshan <laijs@...ux.alibaba.com>

In non-xenpv, the sp0 is the trampoline stack, and sync_regs() is
called on non-xenpv only since error_entry is not called on xenpv, so
the stack must be the trampoline stack or one of the IST stack and the
check in sync_regs() is unneeded.

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/kernel/traps.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index a6c0bc9ee36f..63cdb7aedd67 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -708,13 +708,15 @@ DEFINE_IDTENTRY_RAW(exc_int3)
 /*
  * Help handler running on a per-cpu (IST or entry trampoline) stack
  * to switch to the normal thread stack if the interrupted code was in
- * user mode. The actual stack switch is done in entry_64.S
+ * user mode.  The actual stack switch is done in entry_64.S.  It is not
+ * called from XENPV and the caller has ensured it is not on the thread
+ * stack.
  */
 asmlinkage __visible noinstr struct pt_regs *sync_regs(struct pt_regs *eregs)
 {
 	struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
-	if (regs != eregs)
-		*regs = *eregs;
+
+	*regs = *eregs;
 	return regs;
 }
 
-- 
2.19.1.6.gb485710b