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Date:   Tue, 24 Oct 2023 14:43:47 +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 5/9] x86/percpu/64: Use relative percpu offsets

On Mon, Oct 23, 2023 at 11:17 PM Brian Gerst <brgerst@...il.com> wrote:
>
> The percpu section is currently linked at virtual address 0, because
> older compilers hardcoded the stack protector canary value at a fixed
> offset from the start of the GS segment.  Now that the canary is a
> normal percpu variable, the percpu section can be linked normally.
> This means that x86-64 will calculate percpu offsets like most other
> architectures, as the delta between the initial percpu address and the
> dynamically allocated memory.
>
> Signed-off-by: Brian Gerst <brgerst@...il.com>
> ---
>  arch/x86/kernel/head_64.S      |  6 ------
>  arch/x86/kernel/setup_percpu.c | 12 ++----------
>  arch/x86/kernel/vmlinux.lds.S  | 24 +-----------------------
>  arch/x86/tools/relocs.c        | 10 +++-------
>  arch/x86/xen/xen-head.S        |  6 ------
>  init/Kconfig                   |  2 +-
>  6 files changed, 7 insertions(+), 53 deletions(-)
>
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index f2453eb38417..b35f74e58dd7 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -72,14 +72,8 @@ SYM_CODE_START_NOALIGN(startup_64)
>
>         /* Setup GSBASE to allow stack canary access for C code */
>         movl    $MSR_GS_BASE, %ecx
> -#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
> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index 2c97bf7b56ae..8707dd07b9ce 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -23,18 +23,10 @@
>  #include <asm/cpumask.h>
>  #include <asm/cpu.h>
>
> -#ifdef CONFIG_X86_64
> -#define BOOT_PERCPU_OFFSET ((unsigned long)__per_cpu_load)
> -#else
> -#define BOOT_PERCPU_OFFSET 0
> -#endif
> -
> -DEFINE_PER_CPU_READ_MOSTLY(unsigned long, this_cpu_off) = BOOT_PERCPU_OFFSET;
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, this_cpu_off);
>  EXPORT_PER_CPU_SYMBOL(this_cpu_off);
>
> -unsigned long __per_cpu_offset[NR_CPUS] __ro_after_init = {
> -       [0 ... NR_CPUS-1] = BOOT_PERCPU_OFFSET,
> -};
> +unsigned long __per_cpu_offset[NR_CPUS] __ro_after_init;
>  EXPORT_SYMBOL(__per_cpu_offset);
>
>  /*
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index c87dc8de2084..8d8eb4d9ff9d 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -102,12 +102,6 @@ jiffies = jiffies_64;
>  PHDRS {
>         text PT_LOAD FLAGS(5);          /* R_E */
>         data PT_LOAD FLAGS(6);          /* RW_ */
> -#ifdef CONFIG_X86_64
> -#ifdef CONFIG_SMP
> -       percpu PT_LOAD FLAGS(6);        /* RW_ */
> -#endif
> -       init PT_LOAD FLAGS(7);          /* RWE */
> -#endif
>         note PT_NOTE FLAGS(0);          /* ___ */
>  }
>
> @@ -223,21 +217,7 @@ SECTIONS
>                 __init_begin = .; /* paired with __init_end */
>         }
>
> -#if defined(CONFIG_X86_64) && defined(CONFIG_SMP)
> -       /*
> -        * percpu offsets are zero-based on SMP.  PERCPU_VADDR() changes the
> -        * output PHDR, so the next output section - .init.text - should
> -        * start another segment - init.
> -        */
> -       PERCPU_VADDR(INTERNODE_CACHE_BYTES, 0, :percpu)
> -       ASSERT(SIZEOF(.data..percpu) < CONFIG_PHYSICAL_START,
> -              "per-CPU data too large - increase CONFIG_PHYSICAL_START")
> -#endif
> -
>         INIT_TEXT_SECTION(PAGE_SIZE)
> -#ifdef CONFIG_X86_64
> -       :init
> -#endif
>
>         /*
>          * Section for code used exclusively before alternatives are run. All
> @@ -367,9 +347,7 @@ SECTIONS
>                 EXIT_DATA
>         }
>
> -#if !defined(CONFIG_X86_64) || !defined(CONFIG_SMP)
>         PERCPU_SECTION(INTERNODE_CACHE_BYTES)
> -#endif
>
>         . = ALIGN(PAGE_SIZE);
>
> @@ -507,7 +485,7 @@ SECTIONS
>   * Per-cpu symbols which need to be offset from __per_cpu_load
>   * for the boot processor.
>   */
> -#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
> +#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x)
>  INIT_PER_CPU(gdt_page);
>  INIT_PER_CPU(irq_stack_backing_store);
>
> diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
> index 3ccd9d4fcf9c..01efbfdd3eb3 100644
> --- a/arch/x86/tools/relocs.c
> +++ b/arch/x86/tools/relocs.c
> @@ -815,12 +815,7 @@ static void percpu_init(void)
>   */
>  static int is_percpu_sym(ElfW(Sym) *sym, const char *symname)
>  {
> -       int shndx = sym_index(sym);
> -
> -       return (shndx == per_cpu_shndx) &&
> -               strcmp(symname, "__init_begin") &&
> -               strcmp(symname, "__per_cpu_load") &&
> -               strncmp(symname, "init_per_cpu_", 13);
> +       return 0;
>  }
>
>
> @@ -1043,7 +1038,8 @@ static int cmp_relocs(const void *va, const void *vb)
>
>  static void sort_relocs(struct relocs *r)
>  {
> -       qsort(r->offset, r->count, sizeof(r->offset[0]), cmp_relocs);
> +       if (r->count)
> +               qsort(r->offset, r->count, sizeof(r->offset[0]), cmp_relocs);
>  }
>
>  static int write32(uint32_t v, FILE *f)
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 9ce0d9d268bb..c1d9c92b417a 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -57,14 +57,8 @@ SYM_CODE_START(startup_xen)
>          * the per cpu areas are set up.
>          */
>         movl    $MSR_GS_BASE,%ecx
> -#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

Again, please note there is another access to $MSR_GS_BASE in
/arch/x86/platform/pvh/head.S that should be reviewed.

Uros.

> diff --git a/init/Kconfig b/init/Kconfig
> index 6d35728b94b2..1af31b23e376 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1718,7 +1718,7 @@ config KALLSYMS_ALL
>  config KALLSYMS_ABSOLUTE_PERCPU
>         bool
>         depends on KALLSYMS
> -       default X86_64 && SMP
> +       default n
>
>  config KALLSYMS_BASE_RELATIVE
>         bool
> --
> 2.41.0
>

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