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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Ard Biesheuvel <ardb@kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>
Subject: Re: [PATCH 0/8] Remove 64-bit GDT setup in efi_main + doc fixes
Date: Fri, 31 Jan 2020 09:42:34 +0100
Message-ID: <CAKv+Gu8JGC21R+ngSYtVxFDF7m2nvX=Opo1fkvdFSq-Fz_7B_Q@mail.gmail.com> (raw)
In-Reply-To: <20200130200440.1796058-1-nivedita@alum.mit.edu>

Thanks Arvind, this is good stuff.


On Thu, 30 Jan 2020 at 21:04, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> For the 64-bit kernel, we do not need to setup a GDT in efi_main, as the
> next step in the boot, startup_64, will set one up.
>

This seems obvious, but I need a nod from Ingo or Boris before I can take this.

> This series factors out the GDT setup into a separate function and
> restricts it to the 32-bit kernel. The memory allocation for the GDT is
> also changed from allocating a full page via efi_alloc_low to the
> required space (32 bytes) from alloc_pool boot service.
>

Can we go all the way and simply make this

if (IS_ENABLED(CONFIG_X64_32)) {
  static const struct desc_struct desc[] __aligned(8) = {
    {}, /* The first GDT is a dummy. */
    {}, /* Second entry is unused on 32-bit */
    GDT_ENTRY_INIT(0xa09b, 0, 0xfffff), /* __KERNEL_CS */
    GDT_ENTRY_INIT(0xc093, 0, 0xfffff), /* __KERNEL_DS */
  };
  struct desc_ptr gdt = { sizeof(desc) - 1, (unsigned long)desc };

  native_load_gdt(&gdt);
}

(and drop the #defines from eboot.h that we no longer use)

> The final two patches are doc fixes to clarify that the 32-bit EFI
> handover entry point also requires segments/GDT to be setup, and that
> for 64-bit mode we don't have any special segment requirements (the
> 64-bit doc currently is just a copy of the 32-bit doc which isn't
> right).
>
> Arvind Sankar (8):
>   efi/x86: Use C wrapper instead of inline assembly
>   efi/x86: Allocate the GDT pointer on the stack
>   efi/x86: Factor GDT setup code into a function

Given the above, I don't think we need the separate function, but if
we do, please drop the 64-bit code first, otherwise there is much more
churn than necessary.

>   efi/x86: Only setup the GDT for 32-bit kernel
>   efi/x86: Allocate only the required 32 bytes for the GDT
>   efi/x86: Change __KERNEL_{CS,DS} to __BOOT_{CS,DS}
>   Documentation/x86/boot: Clarify segment requirements for EFI handover
>   Documentation/x86/boot: Correct segment requirements for 64-bit boot
>
>  Documentation/x86/boot.rst              |  15 +-
>  arch/x86/boot/compressed/eboot.c        | 174 ++++++++++--------------
>  arch/x86/boot/compressed/efi_thunk_64.S |   4 +-
>  3 files changed, 85 insertions(+), 108 deletions(-)
>
> --
> 2.24.1
>

  parent reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-30 20:04 Arvind Sankar
2020-01-30 20:04 ` [PATCH 1/8] efi/x86: Use C wrapper instead of inline assembly Arvind Sankar
2020-01-30 20:04 ` [PATCH 2/8] efi/x86: Allocate the GDT pointer on the stack Arvind Sankar
2020-01-30 20:04 ` [PATCH 3/8] efi/x86: Factor GDT setup code into a function Arvind Sankar
2020-01-30 20:04 ` [PATCH 4/8] efi/x86: Only setup the GDT for 32-bit kernel Arvind Sankar
2020-01-30 20:04 ` [PATCH 5/8] efi/x86: Allocate only the required 32 bytes for the GDT Arvind Sankar
2020-01-30 20:04 ` [PATCH 6/8] efi/x86: Change __KERNEL_{CS,DS} to __BOOT_{CS,DS} Arvind Sankar
2020-01-30 20:04 ` [PATCH 7/8] Documentation/x86/boot: Clarify segment requirements for EFI handover Arvind Sankar
2020-01-31 19:24   ` Arvind Sankar
2020-01-30 20:04 ` [PATCH 8/8] Documentation/x86/boot: Correct segment requirements for 64-bit boot Arvind Sankar
2020-01-31  8:42 ` Ard Biesheuvel [this message]
2020-01-31  9:31   ` [PATCH 0/8] Remove 64-bit GDT setup in efi_main + doc fixes Ard Biesheuvel
2020-01-31 19:10     ` Arvind Sankar
2020-02-02 17:13 ` [PATCH v2 0/7] x86/efi,boot: GDT handling cleanup/fixes Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 1/7] x86/boot: Remove KEEP_SEGMENTS support Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 2/7] efi/x86: Don't depend on firmware GDT layout Arvind Sankar
2020-02-02 17:54     ` Ard Biesheuvel
2020-02-02 18:18       ` Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 3/7] x86/boot: Reload GDTR after copying to the end of the buffer Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 4/7] x86/boot: Clear direction and interrupt flags in startup_64 Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 5/7] efi/x86: Remove GDT setup from efi_main Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 6/7] x86/boot: GDT limit value should be size - 1 Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 7/7] x86/boot: Micro-optimize GDT loading instructions Arvind Sankar
2020-02-02 18:01   ` [PATCH v2 0/7] x86/efi,boot: GDT handling cleanup/fixes Ard Biesheuvel

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