Re: [Minios-devel] [UNIKRAFT PATCHv4 19/43] plat/kvm: Add link script for Arm64

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Julien Grall <julien.grall@xxxxxxx>
> Sent: 2018年7月10日 21:13
> To: Wei Chen <Wei.Chen@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> simon.kuenzer@xxxxxxxxx
> Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 19/43] plat/kvm: Add link script
> for Arm64
> 
> 
> 
> On 10/07/18 10:28, Wei Chen wrote:
> > Hi Julien,
> 
> Hi Wei,
> 
> >> -----Original Message-----
> >> From: Julien Grall <julien.grall@xxxxxxx>
> >> Sent: 2018年7月9日 4:27
> >> To: Wei Chen <Wei.Chen@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> >> simon.kuenzer@xxxxxxxxx
> >> Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> >> Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 19/43] plat/kvm: Add link
> script
> >> for Arm64
> >>
> >> Hi,
> >>
> >> On 07/06/2018 10:03 AM, Wei Chen wrote:
> >>> This link script is based on x86 version, except following
> >>> differences:
> >>> 1. Arm64 needs DTB to parse devices, and QEMU/KVM will revserve
> >>
> >> s/revserve/reserved/
> >>
> >>>      the first 64KB of RAM as DTB area. In this case, we add a DTB
> >>>      section to this link script.
> >>
> >> Do you mind giving a pointer to the QEMU/KVM layout? But then, do we
> >
> > What did you mean a pointer? A source code link or others?
> 
> A link to anything describing the layout in QEMU source. This is useful
> for the reviewer or anyone in the future to understand where your values
> are taken from.
> 

Ok, that would be good, I will place a source code link in my code comments

> [...]
> 
> >>> + /* Read-write data (uninitialized) */
> >>
> >> The word "uninitialized" is a bit misleading here. In C, this section
> >> should be Zeroed. So there are not really "unitialized".
> >>
> >
> > Without GCC -fzero-initialized-in-bss, this section in image is not zero,
> > It is uninitialized. And here, the uninitialized means the objects in
> > Code are not explicitly initialized to a value. System behavior to
> > initialize BSS section to zero is not in this scope.
> 
> Looking at the man, GCC will put variable that are initialized to 0 in
> BSS unless -fnozero-initialized-in-bss.
> 
> However, I don't see this option passed to GCC in Unikraft code. Did I
> miss anything?
> 

We don't use -fnozero-initialized-in-bss in Unikraft, and I know the bss
section has been initialized to zero:
000000004001f408 <ns_per_tick>:
    4001f408:   00000000        .word   0x00000000
But "uninitialized" here means all statically allocated objects that haven't
been assigned a value in Code will be placed in BSS section. This doesn't mean
this BSS section is uninitialized.

> Cheers,
> 
> --
> Julien Grall
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