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 23:05:39 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, 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 Thu, Oct 12, 2023 at 11:02 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> 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??

Yes, 32-bit targets default to %fs. Please also note a new version
(v2) of the patch that reimplements this part.

Uros.