linux-kernel - [PATCH 4/6] x86/gsseg: move local_irq_save/restore() into asm_load_gs

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Date:   Thu,  6 Oct 2022 08:40:39 -0700
From:   Xin Li <xin3.li@...el.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com
Subject: [PATCH 4/6] x86/gsseg: move local_irq_save/restore() into asm_load_gs_index()

From: "H. Peter Anvin (Intel)" <hpa@...or.com>

The need to disable/enable interrupts around asm_load_gs_index() is a
consequence of the implementation. Prepare for using the LKGS
instruction, which is locally atomic and therefore doesn't need
interrupts disabled.

Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
Signed-off-by: Xin Li <xin3.li@...el.com>
---
 arch/x86/entry/entry_64.S            | 26 +++++++++++++++++++++-----
 arch/x86/include/asm/special_insns.h |  4 ----
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e0c48998d2fb..cc6ba6672418 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -778,19 +778,35 @@ SYM_CODE_END(common_interrupt_return)
 _ASM_NOKPROBE(common_interrupt_return)
 
 /*
- * Reload gs selector with exception handling
+ * Reload gs selector with exception handling. This is used only on
+ * native, so using swapgs, pushf, popf, cli, sti, ... directly is fine.
+ *
  *  di:  new selector
+ * rax:  scratch register
  *
  * Is in entry.text as it shouldn't be instrumented.
+ *
+ * Note: popf is slow, so use pushf to read IF and then execute cli/sti
+ * if necessary.
  */
 SYM_FUNC_START(asm_load_gs_index)
 	FRAME_BEGIN
+	pushf
+	pop	%rax
+	andl	$X86_EFLAGS_IF, %eax	/* Interrupts enabled? */
+	jz	1f
+	cli
+1:
 	swapgs
 .Lgs_change:
 	ANNOTATE_NOENDBR // error_entry
 	movl	%edi, %gs
 2:	ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
 	swapgs
+	testl	%eax, %eax
+	jz	3f
+	sti
+3:
 	FRAME_END
 	RET
 
@@ -799,12 +815,12 @@ SYM_FUNC_START(asm_load_gs_index)
 	swapgs					/* switch back to user gs */
 .macro ZAP_GS
 	/* This can't be a string because the preprocessor needs to see it. */
-	movl $__USER_DS, %eax
-	movl %eax, %gs
+	movl $__USER_DS, %edi
+	movl %edi, %gs
 .endm
 	ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
-	xorl	%eax, %eax
-	movl	%eax, %gs
+	xorl	%edi, %edi
+	movl	%edi, %gs
 	jmp	2b
 
 	_ASM_EXTABLE(.Lgs_change, .Lbad_gs)
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index a71d0e8d4684..6de00dec6564 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -124,11 +124,7 @@ extern asmlinkage void asm_load_gs_index(u16 selector);
 
 static inline void native_load_gs_index(unsigned int selector)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
 	asm_load_gs_index(selector);
-	local_irq_restore(flags);
 }
 
 static inline unsigned long __read_cr4(void)
-- 
2.34.1