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Date: Thu, 26 Oct 2023 20:28:00 +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 v2 06/11] x86/percpu/64: Remove fixed_percpu_data
On Thu, Oct 26, 2023 at 6:01 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>
Reviewed-by: Uros Bizjak <ubizjak@...il.com>
> ---
> 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
>
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