Re: [Xen-devel] [PATCH v2 5/6] multiboot2: Add support for relocatable images

[Vladimir 'Ï-coder/phcoder' Serbinenko <phcoder@xxxxxxxxx>]

On 12.11.2015 14:15, Daniel Kiper wrote:
> On Tue, Nov 10, 2015 at 04:23:46PM +0100, Vladimir 'phcoder' Serbinenko wrote:
>> Le 10 nov. 2015 3:52 PM, "Daniel Kiper" <daniel.kiper@xxxxxxxxxx> a Ãcrit :
>>> On Mon, Nov 09, 2015 at 09:08:35PM +0100, Vladimir 'Ï-coder/phcoder'
>> Serbinenko wrote:
>>>> On 20.07.2015 16:35, Daniel Kiper wrote:
>>>>> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
>>>> What is handling the actual relocation?
>>>
>>> In Xen we get image offset from GRUB2 and using this value we calculate
>>> relative addresses. We are using %fs for it.
>>>
>>>> Why doesn't this patch include support for ELF relocations?
>>>
>>> This is another option. We considered it too. However, in our case
>>> relative addressing looks simpler then ELF relocations.
>> How is it simpler than to have a fully relocated binary when you start?
> 
> Xen ELF file does not have relocations.
> 
>> How do you pass the offset?
> 
> Via MULTIBOOT_TAG_TYPE_BASE_ADDR tag.
> 
>> Does xen have any relocation entries?
> 
> No.
> 
Can we then settle on using your interface but saying that bootloader
has to execute all ELF relocations? Since you don't have them, you
wouldn't be affected by the change but it would allow easy creation of
relocatable binaries. Feel free in the first patch just to have a check
for absence of relocation entries. x86-64 has only about 5 relocations
we care about so should be easy to implement. We can reuse code in dl.c.
>> Was such xen already released? Just looking for how it should
>> be in perspective and how to get there
> 
> We agreed (in Xen community) that we would like to have multiboot2 protocol
> with additional features set in stone at first. So, this patch series just
> propose some changes which are required by Xen but they are not used by any
> Xen release yet. Additionally, we want that these extensions are quite generic
> and could be used by other projects if they need them too.
> 
> Daniel
>

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