Re: [Xen-devel] [PATCH v2 8/8] efi: drop original xen.efi code and build mechanism

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Wed, Jul 04, 2018 at 09:34:09AM -0600, Jan Beulich wrote:
> >>> On 04.07.18 at 16:25, <daniel.kiper@xxxxxxxxxx> wrote:
> > On Thu, Jun 28, 2018 at 07:51:52AM -0600, Jan Beulich wrote:
> >> >>> On 19.06.18 at 16:35, <daniel.kiper@xxxxxxxxxx> wrote:
> >> > Then rename xen.mb.efi to xen.efi and drop all related
> >> > differentiators in the code.
> >>
> >> For this you'll first of all need to convince me that the binary you build 
> >> is
> >> a drop-in replacement for xen.efi. As noted in the replies to earlier
> >> patches, I'm getting the impression of this not being the case. A further
> >> hint towards this is the outright deletion of xen/arch/x86/efi/mkreloc.c:
> >> How is the Xen image going to be relocated that way, when loaded from
> >> the EFI shell or boot loader?
> >
> > It works because all addressing is relative to %rip. To be precise, new
> > xen.efi, earlier xen.mb.efi, contains exactly the same code as ELF does.
> > So, if ELF works without any relocations why PE should not. Especially on
> > x86-64. Additionally, my tests showed that in general UEFI implementations
> > just require Base Relocation Table entry in PE Data Directories to relocate
> > the image.
>
> The thing I'm missing in the series is the generation of the relocations to
> go into the section pointed to by this Data Directory entry.

There is no such thing because xen.mb.efi does not need that.

> > Even it can be empty. As it is in current patchset.
>
> No, it can't be empty. Just try removing the relocations from xen.efi and
> see what you get.

I am pretty sure that current xen.efi will not work without .reloc section.
However, xen.mb.efi works without any issue. So, I am not sure where is the
problem. +/- fake .reloc entry which I was talking about earlier.

> > Though I am
> > afraid about more picky UEFI stuff and considering addition of at least
> > one .reloc entry, fake one, as Linux kernel does. And this rises another
> > question: should not we add .bss section into PE header? Right now it is
> > embedded/hidden in PE .text section.
>
> My xen.efi does have a .bss section.

It is completely valid to have BSS embedded in .text section or living
as a separate entity in PE section table. So, the question is: which one
do we prefer? Currently existing xen.mb.efi layout is similar to ELF layout.

Daniel

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