Re: [Xen-devel] [PATCH v9 06/13] efi: create new early memory allocator

[Julien Grall <julien.grall@xxxxxxx>]

Hello Jan,

On 12/10/2016 12:45, Jan Beulich wrote:
> > > > On 11.10.16 at 15:39, <julien.grall@xxxxxxx> wrote:
> > On 06/10/16 13:21, Jan Beulich wrote:
> > > > > > On 05.10.16 at 20:30, <julien.grall@xxxxxxx> wrote:
> > > > On 30/09/2016 02:46, Jan Beulich wrote:
> > > > > > > > On 29.09.16 at 23:42, <daniel.kiper@xxxxxxxxxx> wrote:
> > > > > > +#else
> > > > > > +static void __init free_ebmalloc_unused_mem(void)
> > > > > > +{
> > > > > > +}
> > > > > > +#endif
> > > > > Did you build test this for ARM? The function ought to be unused,
> > > > > as ...
> > > > >
> > > > > > @@ -1251,6 +1301,8 @@ void __init efi_init_memory(void)
> > > > > >      } *extra, *extra_head = NULL;
> > > > > >  #endif
> > > > > >
> > > > > > +    free_ebmalloc_unused_mem();
> > > > > ... the whole function here doesn't get built on ARM.
> > > > >
> > > > > Julien - we're still awaiting your input on general aspects here.
> > > > efi_init_memory would need to be called during Xen boot on ARM. I am not
> > > > sure where as I we don't yet have runtime support on ARM.
> > > >
> > > > Other than that, the patch looks good to me.
> > > But that wasn't the question. My goal is to have as little code
> > > inside #ifndef CONFIG_ARM as possible, and hence I'd like to have
> > > as much of this new code as possible outside of such conditionals.
> > > So the question really is whether that alternative approach would
> > > be fine with you, or what problems you might see.
> > I am not sure to get it. The current approach looks good to me, however,
> > the implementation should not be exposed to ARM until all the TODOs
> > mentioned by Daniel are fixed.
> Which is precisely the opposite of what I'm aiming at. Once again:
> Don't you think it is desirable to keep the #ifndef CONFIG_ARM
> instances to cover as little code as possible? Not all of the named
> TODOs really need to be addressed in order to compile most of
> what comprises this new allocator; in fact none of them really
> needs addressing:
> - if the size estimation turns out to low once ARM starts actually
>   using this, let's just bump it (perhaps by making it a per-arch
>   constant),
> - if the section chosen needs to be different (which it really
>   shouldn't be), let's simply adjust it,
If we keep the section in BSS, then we really need to move the 
initialization of BSS earlier.
This TODO really needs to be fixed now otherwise it will be a nightmare 
to debug later on.
> - as we've already figured there's no need for the stub
>   free_ebmalloc_unused_mem() right now anyway.
But we would need to call free_ebmalloc_unused_mem from somewhere. The 
idea to not expose the early memory allocator on ARM is avoid to have an 
implementation with may not fully work on ARM because of known missing 
pieces.
> And then (as another alternative) we have the option of ARM
> simply defining EBMALLOC_SIZE to zero for the time being. That
> would eliminate the need to actually call free_ebmalloc_unused_mem()
> and turn the other two items into non-issues.
>
> > I would be happy to review any patches addressing the TODOs.
> This, I'm sorry, gets me to raise another question: When is this
> finally going to happen? Shared EFI code was introduced for 4.5,
> and we're now looking to release 4.8 still with runtime support
> unimplemented on ARM. How much longer is this going to take?
Xen is a community project, features are added by contributors when they 
need it. I personally don't have any bandwidth to work on EFI runtimes 
services at the moment (note that the item is in my TODO list as a low 
priority).
I welcome any contribution on EFI runtime support for ARM and will be 
happy to review any series.
Cheers,

--
Julien Grall

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