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Subject: Re: [PATCH V3] LoongArch: Add efistub booting support
To:     Huacai Chen <chenhuacai@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>
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Cc:     Xi Ruoyao <xry111@xry111.site>,
        Huacai Chen <chenhuacai@loongson.cn>,
        Arnd Bergmann <arnd@arndb.de>, loongarch@lists.linux.dev,
        linux-arch <linux-arch@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Guo Ren <guoren@kernel.org>, Xuerui Wang <kernel@xen0n.name>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-efi <linux-efi@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
From:   Youling Tang <tangyouling@loongson.cn>
Message-ID: <8319b9d4-960c-e706-468a-cb58bef6df8c@loongson.cn>
Date:   Mon, 5 Sep 2022 12:34:29 +0800
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Hi, Ard & Huacai

On 09/05/2022 11:50 AM, Huacai Chen wrote:
> Hi, Ard,
>
> On Mon, Sep 5, 2022 at 5:59 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>>
>> On Sun, 4 Sept 2022 at 15:24, Huacai Chen <chenhuacai@kernel.org> wrote:
>>>
>>> Hi, Ard,
>>>
>>> On Thu, Sep 1, 2022 at 6:40 PM Huacai Chen <chenhuacai@kernel.org> wrote:
>>>>
>>>> Hi, Ard,
>>>>
>>>> On Sat, Aug 27, 2022 at 3:14 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>>>>>
>>>>> On Sat, 27 Aug 2022 at 06:41, Xi Ruoyao <xry111@xry111.site> wrote:
>>>>>>
>>>>>> Tested V3 with the magic number check manually removed in my GRUB build.
>>>>>> The system boots successfully.  I've not tested Arnd's zBoot patch yet.
>>>>>
>>>>> I am Ard not Arnd :-)
>>>>>
>>>>> Please use this branch when testing the EFI decompressor:
>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=efi-decompressor-v4
>>>> The root cause of LoongArch zboot boot failure has been found, it is a
>>>> binutils bug, latest toolchain with the below patch can solve the
>>>> problem.
>>>>
>>>> diff --git a/bfd/elfnn-loongarch.c b/bfd/elfnn-loongarch.c
>>>> index 5b44901b9e0..fafdc7c7458 100644
>>>> --- a/bfd/elfnn-loongarch.c
>>>> +++ b/bfd/elfnn-loongarch.c
>>>> @@ -2341,9 +2341,10 @@ loongarch_elf_relocate_section (bfd
>>>> *output_bfd, struct bfd_link_info *info,
>>>>      case R_LARCH_SOP_PUSH_PLT_PCREL:
>>>>        unresolved_reloc = false;
>>>>
>>>> -      if (resolved_to_const)
>>>> +      if (!is_undefweak && resolved_to_const)
>>>>          {
>>>>            relocation += rel->r_addend;
>>>> +          relocation -= pc;
>>>>            break;
>>>>          }
>>>>        else if (is_undefweak)
>>>>
>>>>
>>>> Huacai
>>> Now the patch is submitted here:
>>> https://sourceware.org/pipermail/binutils/2022-September/122713.html
>>>
>>
>> Great. Given the severity of this bug, I imagine that building the
>> LoongArch kernel will require a version of binutils that carries this
>> fix.
>>
>> Therefore, i will revert back to the original approach for accessing
>> uncompressed_size, using an extern declaration with an __aligned(1)
>> attribute.
>>
>>> And I have some other questions about kexec: kexec should jump to the
>>> elf entry or the pe entry? I think is the elf entry, because if we
>>> jump to the pe entry, then SVAM will be executed twice (but it should
>>> be executed only once). However, how can we jump to the elf entry if
>>> we use zboot? Maybe it is kexec-tool's responsibility to decompress
>>> the zboot kernel image?
>>>
>>
>> Yes, very good point. Kexec kernels cannot boot via the EFI entry
>> point, as the boot services will already be shutdown. So the kexec
>> kernel needs to boot via the same entrypoint in the core kernel that
>> the EFI stub calls when it hands over.
>>
>> For the EFI zboot image in particular, we will need to teach kexec how
>> to decompress them. The zboot image has a header that
>> a) describes it as a EFI linux zimg
>> b) describes the start and end offset of the compressed payload
>> c) describes which compression algorithm was used.
>>
>> This means that any non-EFI loader (including kexec) should be able to
>> extract the inner PE/COFF image and decompress it. For arm64 and
>> RISC-V, this is sufficient as the EFI and raw images are the same. For
>> LoongArch, I suppose it means we need a way to enter the core kernel
>> directly via the entrypoint that the EFI stub uses when handing over
>> (and pass the original DT argument so the kexec kernel has access to
>> the EFI and ACPI firmware tables)
> OK, then is this implementation [1] acceptable? I remember that you
> said the MS-DOS header shouldn't contain other information, so I guess
> this is unacceptable?
>
> [1] https://lore.kernel.org/loongarch/c4dbb14a-5580-1e47-3d15-5d2079e88404@loongson.cn/T/#mb8c1dc44f7fa2d3ef638877f0cd3f958f0be96ad

Modifications to the MS-DOS header refer to the arm64 and riscv
implementations [1], and to provide the necessary information to
kexec-tools[2] when loading uncompressed efi images.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0f327f2aaad6a87356cbccfa390d4d3b64d0d3b6
[2] 
https://github.com/horms/kexec-tools/blob/main/kexec/arch/arm64/image-header.h

Thanks,
Youling

>
> Huacai
>