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Date:   Tue, 24 Oct 2023 14:33:08 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Brian Gerst <brgerst@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/9] x86/percpu/64: Remove fixed_percpu_data

On Mon, Oct 23, 2023 at 11:17 PM Brian Gerst <brgerst@...il.com> wrote:
>
> Now that the stack protector canary value is a normal percpu variable,
> fixed_percpu_data is unused and can be removed.
>
> Signed-off-by: Brian Gerst <brgerst@...il.com>
> ---
>  arch/x86/include/asm/processor.h | 13 +++++--------
>  arch/x86/kernel/cpu/common.c     |  4 ----
>  arch/x86/kernel/head_64.S        | 12 ++++++------
>  arch/x86/kernel/vmlinux.lds.S    |  6 ------
>  arch/x86/tools/relocs.c          |  1 -
>  arch/x86/xen/xen-head.S          | 12 ++++++++----
>  6 files changed, 19 insertions(+), 29 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 04371f60e3c6..48c31b8e3e72 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -393,16 +393,13 @@ struct irq_stack {
>  } __aligned(IRQ_STACK_SIZE);
>
>  #ifdef CONFIG_X86_64
> -struct fixed_percpu_data {
> -       char            gs_base[40];
> -};
> -
> -DECLARE_PER_CPU_FIRST(struct fixed_percpu_data, fixed_percpu_data) __visible;
> -DECLARE_INIT_PER_CPU(fixed_percpu_data);
> -
>  static inline unsigned long cpu_kernelmode_gs_base(int cpu)
>  {
> -       return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu);
> +#ifdef CONFIG_SMP
> +       return per_cpu_offset(cpu);
> +#else
> +       return 0;
> +#endif
>  }
>
>  extern asmlinkage void entry_SYSCALL32_ignore(void);
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index f9c8bd27b642..a44fd3ad460e 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2051,10 +2051,6 @@ DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
>  EXPORT_PER_CPU_SYMBOL(pcpu_hot);
>
>  #ifdef CONFIG_X86_64
> -DEFINE_PER_CPU_FIRST(struct fixed_percpu_data,
> -                    fixed_percpu_data) __aligned(PAGE_SIZE) __visible;
> -EXPORT_PER_CPU_SYMBOL_GPL(fixed_percpu_data);
> -
>  static void wrmsrl_cstar(unsigned long val)
>  {
>         /*
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 3dcabbc49149..f2453eb38417 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -72,9 +72,14 @@ SYM_CODE_START_NOALIGN(startup_64)
>
>         /* Setup GSBASE to allow stack canary access for C code */
>         movl    $MSR_GS_BASE, %ecx
> -       leaq    INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> +#ifdef CONFIG_SMP
> +       leaq    __per_cpu_load(%rip), %rdx
>         movl    %edx, %eax
>         shrq    $32,  %rdx
> +#else
> +       xorl    %eax, %eax
> +       xorl    %edx, %edx
> +#endif
>         wrmsr
>
>         call    startup_64_setup_env
> @@ -345,15 +350,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>
>         /* Set up %gs.
>          *
> -        * The base of %gs always points to fixed_percpu_data. If the
> -        * stack protector canary is enabled, it is located at %gs:40.
>          * Note that, on SMP, the boot cpu uses init data section until
>          * the per cpu areas are set up.
>          */
>         movl    $MSR_GS_BASE,%ecx
> -#ifndef CONFIG_SMP
> -       leaq    INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> -#endif
>         movl    %edx, %eax
>         shrq    $32, %rdx
>         wrmsr
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 54a5596adaa6..c87dc8de2084 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -509,14 +509,8 @@ SECTIONS
>   */
>  #define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
>  INIT_PER_CPU(gdt_page);
> -INIT_PER_CPU(fixed_percpu_data);
>  INIT_PER_CPU(irq_stack_backing_store);
>
> -#ifdef CONFIG_SMP
> -. = ASSERT((fixed_percpu_data == 0),
> -           "fixed_percpu_data is not at start of per-cpu area");
> -#endif
> -
>  #ifdef CONFIG_CPU_UNRET_ENTRY
>  . = ASSERT((retbleed_return_thunk & 0x3f) == 0, "retbleed_return_thunk not cacheline-aligned");
>  #endif
> diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
> index d30949e25ebd..3ccd9d4fcf9c 100644
> --- a/arch/x86/tools/relocs.c
> +++ b/arch/x86/tools/relocs.c
> @@ -811,7 +811,6 @@ static void percpu_init(void)
>   *     __per_cpu_load
>   *
>   * The "gold" linker incorrectly associates:
> - *     init_per_cpu__fixed_percpu_data
>   *     init_per_cpu__gdt_page
>   */
>  static int is_percpu_sym(ElfW(Sym) *sym, const char *symname)
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index a0ea285878db..9ce0d9d268bb 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -53,14 +53,18 @@ SYM_CODE_START(startup_xen)
>
>         /* Set up %gs.
>          *
> -        * The base of %gs always points to fixed_percpu_data.  If the
> -        * stack protector canary is enabled, it is located at %gs:40.
>          * Note that, on SMP, the boot cpu uses init data section until
>          * the per cpu areas are set up.
>          */
>         movl    $MSR_GS_BASE,%ecx
> -       movq    $INIT_PER_CPU_VAR(fixed_percpu_data),%rax
> -       cdq
> +#ifdef CONFIG_SMP
> +       leaq    __per_cpu_load(%rip), %rdx
> +       movl    %edx, %eax
> +       shrq    $32, %rdx
> +#else
> +       xorl    %eax, %eax
> +       xorl    %edx, %edx
> +#endif
>         wrmsr
>
>         mov     %rsi, %rdi

Please note there is another access to $MSR_GS_BASE in
/arch/x86/platform/pvh/head.S around line 98. Should this be fixed,
too?

Uros.

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