linux-kernel - Re: [v2 PATCH] RISC-V: Add a PE/COFF compliant Image header.

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Date:   Mon, 13 May 2019 18:13:18 -0700
From:   Atish Patra <atish.patra@....com>
To:     Paul Walmsley <paul.walmsley@...ive.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Zong Li <zong@...estech.com>,
        "merker@...ian.org" <merker@...ian.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [v2 PATCH] RISC-V: Add a PE/COFF compliant Image header.

On 5/13/19 5:40 PM, Paul Walmsley wrote:
> On Mon, 13 May 2019, Atish Patra wrote:
> 
>> On 5/13/19 5:09 PM, Paul Walmsley wrote:
>>
>>> What are the semantics of those reserved fields?
>>
>> +struct riscv_image_header {
>> +	u32 code0;
>> +	u32 code1;
>> +	u64 text_offset;
>> +	u64 image_size;
>> +	u64 res1;
>> +	u64 res2;
>> +	u64 res3;
>> +	u64 magic;
>> +	u32 res4; ---> We can use this for versioning when required
>> +	u32 res5; ---> This is reserved for PE/COFF header
>> +};
> 
> I saw that in your patch.  The problem is that this doesn't describe what
> other software might expect in those fields.  Can anything at all be
> placed in those reserved fields?
> 

Yes. The reserved fields can be used for anything that boot loaders and 
Linux kernel can agree with each other. If you look at the ARM64, they 
have "Informative flags" in place of res1.

>>>> Do we need to add it now or add it later when we actually need a version
>>>> number. My preference is to add it later based on requirement.
>>>
>>> If it isn't added now, how would bootloaders know whether it was there or
>>> not?
>>>
>>>
>> Here is the corresponding U-Boot Patch
>> https://patchwork.ozlabs.org/patch/1096087/
>>
>> Currently, boot loader doesn't care about versioning. Since we are updating a
>> reserved field, offsets will not change. If a boot loader want to use the
>> versioning, it should be patched along with the kernel patch.
>>
>> Any other boot loader that doesn't care about the version, it can continue to
>> do so without any change.
>>
>> My idea is to enable the minimum required fields in this patch and keep
>> everything else as reserved so that it can be amended in future as required.
> 
> If those fields really are reserved for implementors to do whatever they
> want with them, then that might be a reasonable approach.  That seems
> unlikely, however, since specification authors usually reserve the right
> to use reserved fields for their own purposes in later versions.
> 
Technically, we are just implementing the "DOS" header part of PE/COFF 
format for now. It only mandates a magic string "MZ" at the top and a 
32bit value at offset 0x3c tells us offset of PE/COFF header in image.
Anything in between is implementation specific.

For example, it will be updated to support EFI stub as described in the 
commit text,
"In order to support EFI stub, code0 should be replaced with "MZ" magic 
string and res5(at offset 0x3c) should point to the rest of the PE/COFF 
header (which will be added during EFI support)."

Regards,
Atish
> 
> - Paul
>