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Date:   Thu, 26 Sep 2019 12:18:36 -0700
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Subject: Re: [RFC PATCH] x86/doc/boot_protocol: Correct the description of "reloc"
To:     Cao jin <caoj.fnst@cn.fujitsu.com>, Ingo Molnar <mingo@kernel.org>
CC:     x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, corbet@lwn.net
From:   hpa@zytor.com
Message-ID: <A44CE024-DEB0-476C-B71B-883307952F10@zytor.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On September 26, 2019 1:20:02 AM PDT, Cao jin <caoj=2Efnst@cn=2Efujitsu=2Ec=
om> wrote:
>On 9/26/19 3:58 PM, hpa@zytor=2Ecom wrote:
>> On September 26, 2019 12:55:51 AM PDT, Cao jin
><caoj=2Efnst@cn=2Efujitsu=2Ecom> wrote:
>>> On 9/26/19 2:01 PM, Ingo Molnar wrote:
>>>> * Cao jin <caoj=2Efnst@cn=2Efujitsu=2Ecom> wrote:
>>>>
>>>>> The fields marked with (reloc) actually are not dedicated for
>>> writing,
>>>>> but communicating info for relocatable kernel with boot loaders=2E
>For
>>>>> example:
>>>>>
>>>>>     =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D    =3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D
>>>>>     Field name:     pref_address
>>>>>     Type:           read (reloc)
>>>>>     Offset/size:    0x258/8
>>>>>     Protocol:       2=2E10+
>>>>>     =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D    =3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D
>>>>>
>>>>>     =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D    =3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>>>>     Field name:     code32_start
>>>>>     Type:           modify (optional, reloc)
>>>>>     Offset/size:    0x214/4
>>>>>     Protocol:       2=2E00+
>>>>>     =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D    =3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>>>>
>>>>> Signed-off-by: Cao jin <caoj=2Efnst@cn=2Efujitsu=2Ecom>
>>>>> ---
>>>>> Unless I have incorrect non-native understanding for "fill in", I
>>> think
>>>>> this is inaccurate=2E
>>>>>
>>>>>  Documentation/x86/boot=2Erst | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/Documentation/x86/boot=2Erst
>b/Documentation/x86/boot=2Erst
>>>>> index 08a2f100c0e6=2E=2Ea611bf04492d 100644
>>>>> --- a/Documentation/x86/boot=2Erst
>>>>> +++ b/Documentation/x86/boot=2Erst
>>>>> @@ -243,7 +243,7 @@ bootloader ("modify")=2E
>>>>> =20
>>>>>  All general purpose boot loaders should write the fields marked
>>>>>  (obligatory)=2E  Boot loaders who want to load the kernel at a
>>>>> -nonstandard address should fill in the fields marked (reloc);
>other
>>>>> +nonstandard address should consult with the fields marked
>(reloc);
>>> other
>>>>>  boot loaders can ignore those fields=2E
>>>>> =20
>>>>>  The byte order of all fields is littleendian (this is x86, after
>>> all=2E)
>>>>
>>>> Well, this documentation is written from the point of view of a=20
>>>> *bootloader*, not the kernel=2E So the 'fill in' says that the
>>> bootloader=20
>>>> should write those fields - which is correct, right?
>>>>
>>>
>>> Take pref_address or relocatable_kernel for example, they have type:
>>> read (reloc), does boot loader need to write them? I don't see grub
>>> does
>>> this at least=2E
>>=20
>> Read means the boot later reads them=2E
>>=20
>
>Sorry I don't know what is going wrong in my mind=2E For me, if
>pref_address has "read (reloc)", base on the current document, it means
>boot loader will read it and also write it, which is conflicting=2E And
>the purpose of pref_address should just inform boot loader that kernel
>whats itself to be loaded at certain address, it don't want to be
>written=2E

read (reloc) means it is information for the boot loader to read, but that=
 it can ignore it completely if it does not want to relocate the kernel=2E
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E