linux-kernel - Re: [PATCH 1/4] x86/percpu: Use explicit segment registers in lib/cmpxchg{8,16}b

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Date:   Thu, 12 Oct 2023 14:02:12 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] x86/percpu: Use explicit segment registers in lib/cmpxchg{8,16}b_emu.S

On October 12, 2023 9:10:36 AM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
>PER_CPU_VAR macro is intended to be applied to a symbol, it is not
>intended to be used as a selector between %fs and %gs segment
>registers for general operands.
>
>The address to these emulation functions is passed in a register, so
>use explicit segment registers to access percpu variable instead.
>
>Also add a missing function comment to this_cpu_cmpxchg8b_emu.
>
>No functional changes intended.
>
>Cc: Thomas Gleixner <tglx@...utronix.de>
>Cc: Ingo Molnar <mingo@...hat.com>
>Cc: Borislav Petkov <bp@...en8.de>
>Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>Cc: "H. Peter Anvin" <hpa@...or.com>
>Cc: Peter Zijlstra <peterz@...radead.org>
>Signed-off-by: Uros Bizjak <ubizjak@...il.com>
>---
> arch/x86/lib/cmpxchg16b_emu.S | 12 ++++++------
> arch/x86/lib/cmpxchg8b_emu.S  | 30 +++++++++++++++++++++---------
> 2 files changed, 27 insertions(+), 15 deletions(-)
>
>diff --git a/arch/x86/lib/cmpxchg16b_emu.S b/arch/x86/lib/cmpxchg16b_emu.S
>index 6962df315793..2bd8b89bce75 100644
>--- a/arch/x86/lib/cmpxchg16b_emu.S
>+++ b/arch/x86/lib/cmpxchg16b_emu.S
>@@ -23,14 +23,14 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
> 	cli
> 
> 	/* if (*ptr == old) */
>-	cmpq	PER_CPU_VAR(0(%rsi)), %rax
>+	cmpq	%gs:(%rsi), %rax
> 	jne	.Lnot_same
>-	cmpq	PER_CPU_VAR(8(%rsi)), %rdx
>+	cmpq	%gs:8(%rsi), %rdx
> 	jne	.Lnot_same
> 
> 	/* *ptr = new */
>-	movq	%rbx, PER_CPU_VAR(0(%rsi))
>-	movq	%rcx, PER_CPU_VAR(8(%rsi))
>+	movq	%rbx, %gs:(%rsi)
>+	movq	%rcx, %gs:8(%rsi)
> 
> 	/* set ZF in EFLAGS to indicate success */
> 	orl	$X86_EFLAGS_ZF, (%rsp)
>@@ -42,8 +42,8 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
> 	/* *ptr != old */
> 
> 	/* old = *ptr */
>-	movq	PER_CPU_VAR(0(%rsi)), %rax
>-	movq	PER_CPU_VAR(8(%rsi)), %rdx
>+	movq	%gs:(%rsi), %rax
>+	movq	%gs:8(%rsi), %rdx
> 
> 	/* clear ZF in EFLAGS to indicate failure */
> 	andl	$(~X86_EFLAGS_ZF), (%rsp)
>diff --git a/arch/x86/lib/cmpxchg8b_emu.S b/arch/x86/lib/cmpxchg8b_emu.S
>index 49805257b125..b7d68d5e2d31 100644
>--- a/arch/x86/lib/cmpxchg8b_emu.S
>+++ b/arch/x86/lib/cmpxchg8b_emu.S
>@@ -24,12 +24,12 @@ SYM_FUNC_START(cmpxchg8b_emu)
> 	pushfl
> 	cli
> 
>-	cmpl	0(%esi), %eax
>+	cmpl	(%esi), %eax
> 	jne	.Lnot_same
> 	cmpl	4(%esi), %edx
> 	jne	.Lnot_same
> 
>-	movl	%ebx, 0(%esi)
>+	movl	%ebx, (%esi)
> 	movl	%ecx, 4(%esi)
> 
> 	orl	$X86_EFLAGS_ZF, (%esp)
>@@ -38,7 +38,7 @@ SYM_FUNC_START(cmpxchg8b_emu)
> 	RET
> 
> .Lnot_same:
>-	movl	0(%esi), %eax
>+	movl	(%esi), %eax
> 	movl	4(%esi), %edx
> 
> 	andl	$(~X86_EFLAGS_ZF), (%esp)
>@@ -53,18 +53,30 @@ EXPORT_SYMBOL(cmpxchg8b_emu)
> 
> #ifndef CONFIG_UML
> 
>+/*
>+ * Emulate 'cmpxchg8b %fs:(%esi)'
>+ *
>+ * Inputs:
>+ * %esi : memory location to compare
>+ * %eax : low 32 bits of old value
>+ * %edx : high 32 bits of old value
>+ * %ebx : low 32 bits of new value
>+ * %ecx : high 32 bits of new value
>+ *
>+ * Notably this is not LOCK prefixed and is not safe against NMIs
>+ */
> SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
> 
> 	pushfl
> 	cli
> 
>-	cmpl	PER_CPU_VAR(0(%esi)), %eax
>+	cmpl	%fs:(%esi), %eax
> 	jne	.Lnot_same2
>-	cmpl	PER_CPU_VAR(4(%esi)), %edx
>+	cmpl	%fs:4(%esi), %edx
> 	jne	.Lnot_same2
> 
>-	movl	%ebx, PER_CPU_VAR(0(%esi))
>-	movl	%ecx, PER_CPU_VAR(4(%esi))
>+	movl	%ebx, %fs:(%esi)
>+	movl	%ecx, %fs:4(%esi)
> 
> 	orl	$X86_EFLAGS_ZF, (%esp)
> 
>@@ -72,8 +84,8 @@ SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
> 	RET
> 
> .Lnot_same2:
>-	movl	PER_CPU_VAR(0(%esi)), %eax
>-	movl	PER_CPU_VAR(4(%esi)), %edx
>+	movl	%fs:(%esi), %eax
>+	movl	%fs:4(%esi), %edx
> 
> 	andl	$(~X86_EFLAGS_ZF), (%esp)
> 

%fs??