linux-kernel - [PATCH V3 3/6] x86_32/sysenter: switch to the task stack without emptying the entry stack

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Date:   Thu, 28 Jan 2021 00:32:19 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Kees Cook <keescook@...omium.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Brian Gerst <brgerst@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Mike Rapoport <rppt@...nel.org>, Mike Hommey <mh@...ndium.org>,
        Mark Gross <mgross@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Jay Lang <jaytlang@....edu>,
        "Chang S. Bae" <chang.seok.bae@...el.com>
Subject: [PATCH V3 3/6] x86_32/sysenter: switch to the task stack without emptying the entry stack

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

Like the way x86_64 uses the "old" stack, we can save the entry stack
pointer to a register and switch to the task stack.  So that we have
space on the "old" stack to save more things or scratch registers.

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/entry/entry_32.S | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 3b4d1a63d1f0..3e693db0963d 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -905,19 +905,18 @@ SYM_FUNC_START(entry_SYSENTER_32)
 	pushl	%eax
 	BUG_IF_WRONG_CR3 no_user_check=1
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
-	popl	%eax
-	popfl
 
-	/* Stack empty again, switch to task stack */
-	movl	TSS_entry2task_stack(%esp), %esp
+	/* Switch to task stack */
+	movl	%esp, %eax
+	movl	(2*4+TSS_entry2task_stack)(%esp), %esp
 
 .Lsysenter_past_esp:
 	pushl	$__USER_DS		/* pt_regs->ss */
 	pushl	$0			/* pt_regs->sp (placeholder) */
-	pushfl				/* pt_regs->flags (except IF = 0) */
+	pushl	%ss:4(%eax)		/* pt_regs->flags (except IF = 0) */
 	pushl	$__USER_CS		/* pt_regs->cs */
 	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
-	pushl	%eax			/* pt_regs->orig_ax */
+	pushl	%ss:(%eax)		/* pt_regs->orig_ax */
 	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest, stack already switched */
 
 	/*
-- 
2.19.1.6.gb485710b