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From: Uros Bizjak <ubizjak@gmail.com>
Date: Sat, 23 Mar 2024 18:14:15 +0100
Message-ID: <CAFULd4Zw-iCNVoz_+QN2KMOOZex6-DZZDSSN2uGbWvPsJFcJtQ@mail.gmail.com>
Subject: Re: [PATCH v4 09/16] x86/percpu/64: Use relative percpu offsets
To: Brian Gerst <brgerst@gmail.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org, 
	Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, 
	"H . Peter Anvin" <hpa@zytor.com>, David.Laight@aculab.com
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On Fri, Mar 22, 2024 at 5:52=E2=80=AFPM Brian Gerst <brgerst@gmail.com> wro=
te:
>
> 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 does not need to be linked
> at a specific virtual address.  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@gmail.com>

Reviewed-by: Uros Bizjak <ubizjak@gmail.com>

> ---
>  arch/x86/include/asm/processor.h |  6 +++++-
>  arch/x86/kernel/head_64.S        | 19 +++++++++----------
>  arch/x86/kernel/setup_percpu.c   | 12 ++----------
>  arch/x86/kernel/vmlinux.lds.S    | 29 +----------------------------
>  arch/x86/platform/pvh/head.S     |  5 ++---
>  arch/x86/tools/relocs.c          | 10 +++-------
>  arch/x86/xen/xen-head.S          |  9 ++++-----
>  init/Kconfig                     |  2 +-
>  8 files changed, 27 insertions(+), 65 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/proc=
essor.h
> index 946bebce396f..40d6add8ff31 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -396,7 +396,11 @@ 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/head_64.S b/arch/x86/kernel/head_64.S
> index cfbf0486d424..5b2cc711feec 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -68,11 +68,14 @@ SYM_CODE_START_NOALIGN(startup_64)
>         /* Set up the stack for verify_cpu() */
>         leaq    __top_init_kernel_stack(%rip), %rsp
>
> -       /* Setup GSBASE to allow stack canary access for C code */
> +       /*
> +        * Set up GSBASE.
> +        * Note that, on SMP, the boot cpu uses init data section until
> +        * the per cpu areas are set up.
> +        */
>         movl    $MSR_GS_BASE, %ecx
> -       leaq    INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> -       movl    %edx, %eax
> -       shrq    $32,  %rdx
> +       xorl    %eax, %eax
> +       xorl    %edx, %edx
>         wrmsr
>
>         call    startup_64_setup_gdt_idt
> @@ -359,16 +362,12 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
>         movl %eax,%fs
>         movl %eax,%gs
>
> -       /* Set up %gs.
> -        *
> -        * The base of %gs always points to fixed_percpu_data.
> +       /*
> +        * Set up GSBASE.
>          * 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/setup_percpu.c b/arch/x86/kernel/setup_percp=
u.c
> index b30d6e180df7..1e7be9409aa2 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) =3D 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 =3D {
> -       [0 ... NR_CPUS-1] =3D 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 3509afc6a672..0b152f96c24e 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -99,12 +99,6 @@ const_pcpu_hot =3D pcpu_hot;
>  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);          /* ___ */
>  }
>
> @@ -222,21 +216,7 @@ SECTIONS
>                 __init_begin =3D .; /* 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
> @@ -353,9 +333,7 @@ SECTIONS
>                 EXIT_DATA
>         }
>
> -#if !defined(CONFIG_X86_64) || !defined(CONFIG_SMP)
>         PERCPU_SECTION(INTERNODE_CACHE_BYTES)
> -#endif
>
>         . =3D ALIGN(PAGE_SIZE);
>
> @@ -493,16 +471,11 @@ 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 =3D ABSOLUTE(x) + __per_cpu_lo=
ad
> +#define INIT_PER_CPU(x) init_per_cpu__##x =3D ABSOLUTE(x)
>  INIT_PER_CPU(gdt_page);
>  INIT_PER_CPU(fixed_percpu_data);
>  INIT_PER_CPU(irq_stack_backing_store);
>
> -#ifdef CONFIG_SMP
> -. =3D ASSERT((fixed_percpu_data =3D=3D 0),
> -           "fixed_percpu_data is not at start of per-cpu area");
> -#endif
> -
>  #ifdef CONFIG_MITIGATION_UNRET_ENTRY
>  . =3D ASSERT((retbleed_return_thunk & 0x3f) =3D=3D 0, "retbleed_return_t=
hunk not cacheline-aligned");
>  #endif
> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index 1f1c3230b27b..7e3e07c6170f 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -101,9 +101,8 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
>          * the per cpu areas are set up.
>          */
>         mov $MSR_GS_BASE,%ecx
> -       lea INIT_PER_CPU_VAR(fixed_percpu_data)(%rip), %rdx
> -       mov %edx, %eax
> -       shr $32, %rdx
> +       xor %eax, %eax
> +       xor %edx, %edx
>         wrmsr
>
>         call xen_prepare_pvh
> diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
> index adf11a48ec70..016650ddaf7f 100644
> --- a/arch/x86/tools/relocs.c
> +++ b/arch/x86/tools/relocs.c
> @@ -839,12 +839,7 @@ static void percpu_init(void)
>   */
>  static int is_percpu_sym(ElfW(Sym) *sym, const char *symname)
>  {
> -       int shndx =3D sym_index(sym);
> -
> -       return (shndx =3D=3D per_cpu_shndx) &&
> -               strcmp(symname, "__init_begin") &&
> -               strcmp(symname, "__per_cpu_load") &&
> -               strncmp(symname, "init_per_cpu_", 13);
> +       return 0;
>  }
>
>
> @@ -1068,7 +1063,8 @@ static int cmp_relocs(const void *va, const void *v=
b)
>
>  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_relo=
cs);
>  }
>
>  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 ae4672ea00bb..1796884b727d 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -51,15 +51,14 @@ SYM_CODE_START(startup_xen)
>
>         leaq    __top_init_kernel_stack(%rip), %rsp
>
> -       /* Set up %gs.
> -        *
> -        * The base of %gs always points to fixed_percpu_data.
> +       /*
> +        * Set up GSBASE.
>          * 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
> +       xorl    %eax, %eax
> +       xorl    %edx, %edx
>         wrmsr
>
>         mov     %rsi, %rdi
> diff --git a/init/Kconfig b/init/Kconfig
> index f3ea5dea9c85..0f928f82dc7a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1784,7 +1784,7 @@ config KALLSYMS_ALL
>  config KALLSYMS_ABSOLUTE_PERCPU
>         bool
>         depends on KALLSYMS
> -       default X86_64 && SMP
> +       default n
>
>  config KALLSYMS_BASE_RELATIVE
>         bool
> --
> 2.44.0
>