linux-kernel - Re: [PATCH v2] x86/boot/64: Load kernel GDT in early boot

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Date: Tue, 27 Feb 2024 15:57:42 +0100
From: Ard Biesheuvel <ardb@...nel.org>
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] x86/boot/64: Load kernel GDT in early boot

On Mon, 26 Feb 2024 at 23:05, Brian Gerst <brgerst@...il.com> wrote:
>
> Instead of loading a duplicate GDT just for early boot, load the kernel
> GDT from its physical address.
>
> Signed-off-by: Brian Gerst <brgerst@...il.com>
> ---
> v2: Updated to latest tip tree
>

Acked-by: Ard Biesheuvel <ardb@...nel.org>

>  arch/x86/include/asm/desc.h |  1 +
>  arch/x86/kernel/head64.c    | 13 ++-----------
>  2 files changed, 3 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
> index ec95fe44fa3a..62dc9f59ea76 100644
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -46,6 +46,7 @@ struct gdt_page {
>  } __attribute__((aligned(PAGE_SIZE)));
>
>  DECLARE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page);
> +DECLARE_INIT_PER_CPU(gdt_page);
>
>  /* Provide the original GDT */
>  static inline struct desc_struct *get_cpu_gdt_rw(unsigned int cpu)
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 72351c3121a6..fd77a266f29d 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -68,15 +68,6 @@ unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
>  EXPORT_SYMBOL(vmemmap_base);
>  #endif
>
> -/*
> - * GDT used on the boot CPU before switching to virtual addresses.
> - */
> -static struct desc_struct startup_gdt[GDT_ENTRIES] __initdata = {
> -       [GDT_ENTRY_KERNEL32_CS]         = GDT_ENTRY_INIT(DESC_CODE32, 0, 0xfffff),
> -       [GDT_ENTRY_KERNEL_CS]           = GDT_ENTRY_INIT(DESC_CODE64, 0, 0xfffff),
> -       [GDT_ENTRY_KERNEL_DS]           = GDT_ENTRY_INIT(DESC_DATA64, 0, 0xfffff),
> -};
> -
>  #ifdef CONFIG_X86_5LEVEL
>  static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
>  {
> @@ -589,8 +580,8 @@ void __head startup_64_setup_gdt_idt(void)
>         void *handler = NULL;
>
>         struct desc_ptr startup_gdt_descr = {
> -               .address = (unsigned long)&RIP_REL_REF(startup_gdt),
> -               .size    = sizeof(startup_gdt) - 1,
> +               .address = (unsigned long)&RIP_REL_REF(init_per_cpu_var(gdt_page.gdt)),
> +               .size    = GDT_SIZE - 1,
>         };
>
>         /* Load GDT */
>
> base-commit: afb9cce6ad6e808cf659f43925941be96f61b9c9
> --
> 2.43.2
>