Re: [PATCH v2 06/11] x86/percpu/64: Remove fixed_percpu_data

[Uros Bizjak]

On Thu, Oct 26, 2023 at 6:01 PM Brian Gerst <brgerst@xxxxxxxxx> 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@xxxxxxxxx>

Reviewed-by: Uros Bizjak <ubizjak@xxxxxxxxx>

> ---
>  arch/x86/include/asm/processor.h | 13 +++++--------
>  arch/x86/kernel/cpu/common.c     |  4 ----
>  arch/x86/kernel/head_64.S        | 11 ++++++-----
>  arch/x86/kernel/vmlinux.lds.S    |  6 ------
>  arch/x86/platform/pvh/head.S     |  7 ++++++-
>  arch/x86/tools/relocs.c          |  1 -
>  arch/x86/xen/xen-head.S          | 11 ++++++++---
>  7 files changed, 25 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 2b6531d90273..d5a4044dba8f 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 fb8f0371ffc3..32ae76cf4508 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2052,10 +2052,6 @@ EXPORT_PER_CPU_SYMBOL(pcpu_hot);
>  EXPORT_PER_CPU_SYMBOL(const_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 0d94d2a091fe..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
> @@ -345,14 +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.
>          * 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 1239be7cc8d8..e6126cd21615 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -510,14 +510,8 @@ 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/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index be8d973c0528..d215b16bf89f 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -96,9 +96,14 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
>  1:
>         /* Set base address in stack canary descriptor. */
>         mov $MSR_GS_BASE,%ecx
> -       lea INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> +#ifdef CONFIG_SMP
> +       lea __per_cpu_load(%rip), %rdx
>         mov %edx, %eax
>         shr $32, %rdx
> +#else
> +       xor %eax, %eax
> +       xor %edx, %edx
> +#endif
>         wrmsr
>
>         call xen_prepare_pvh
> 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 30f27e757354..9ce0d9d268bb 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -53,13 +53,18 @@ SYM_CODE_START(startup_xen)
>
>         /* Set up %gs.
>          *
> -        * The base of %gs always points to fixed_percpu_data.
>          * 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
> --
> 2.41.0
>