Date:   Mon, 5 Sep 2022 15:34:46 +0800
From:   Youling Tang <tangyouling@...ngson.cn>
To:     Huacai Chen <chenhuacai@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     Xi Ruoyao <xry111@...111.site>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>, loongarch@...ts.linux.dev,
        linux-arch <linux-arch@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3] LoongArch: Add efistub booting support

Hi, Huacai

On 09/05/2022 03:24 PM, Huacai Chen wrote:
> Hi, Ard and Youling,
>
> On Mon, Sep 5, 2022 at 3:02 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>>
>> On Mon, 5 Sept 2022 at 05:51, Huacai Chen <chenhuacai@...nel.org> wrote:
>>>
>>> Hi, Ard,
>>>
>>> On Mon, Sep 5, 2022 at 5:59 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>>>>
>>>> On Sun, 4 Sept 2022 at 15:24, Huacai Chen <chenhuacai@...nel.org> wrote:
>>>>>
>>>>> Hi, Ard,
>>>>>
>>>>> On Thu, Sep 1, 2022 at 6:40 PM Huacai Chen <chenhuacai@...nel.org> wrote:
>>>>>>
>>>>>> Hi, Ard,
>>>>>>
>>>>>> On Sat, Aug 27, 2022 at 3:14 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>>>>>>>
>>>>>>> On Sat, 27 Aug 2022 at 06:41, Xi Ruoyao <xry111@...111.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?
>>>
>>
>> No, this looks reasonable to me. I objected to using magic numbers in
>> the 'pure PE' view of the image, as it does not make sense for a pure
>> PE loader such as GRUB to rely on such metadata.
>>
>> In this case (like on arm64), we are dealing with something else: we
>> need to identify the image to the kernel itself, and here, using the
>> unused space in the MS-DOS header is fine.
>>
>>> [1] https://lore.kernel.org/loongarch/c4dbb14a-5580-1e47-3d15-5d2079e88404@loongson.cn/T/#mb8c1dc44f7fa2d3ef638877f0cd3f958f0be96ad
> OK, then there is no big problem here. And I found that arm64/riscv
> don't need the kernel entry point in the header. I don't know why, but
> I think it implies that a unified layout across architectures is
> unnecessary, and I prefer to put the kernel entry point before
> effective kernel size. :)

The kernel entry point is added because LoongArch has not implemented
purgatory in kexec-tools, so I want to get it from the head through a
simpler method, similar to the elf image operation.

Youling.

>
> Huacai
>
>>>
>>> Huacai

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