Re: [Xen-devel] [PATCH v6 08/15] x86/efi: create new early memory allocator

[Julien Grall <julien.grall@xxxxxxx>]

On 23/09/16 12:35, Daniel Kiper wrote:
> On Fri, Sep 23, 2016 at 12:07:14PM +0100, Julien Grall wrote:
> > On 23/09/16 11:50, Daniel Kiper wrote:
> > > Hi Julien,
> > Hi Daniel,
> >
> > > On Thu, Sep 22, 2016 at 06:07:26PM +0100, Julien Grall wrote:
> > >
> > > [...]
> > >
> > > > > #ifndef CONFIG_ARM
> > > > > /* Whole x86 ebmalloc stuff. */
> > > > > ...
> > > > > #else
> > > > > void __init free_ebmalloc_unused_mem(void)
> > > > > {
> > > > > }
> > > > > #endif
> > > > >
> > > > > and then call free_ebmalloc_unused_mem() from e.g.
> > > > > xen/arch/arm/setup.c:init_done(). Am I right?
> > > > Bear in mind that the EFI loader on ARM is standalone. It cannot
> > > > interact with Xen.
> > > >
> > > > The main goal of the EFI stub is to load the different images on the
> > > > memory and then will jump at the same starting point as when Xen is
> > > > loaded without EFI. So anything in bss will be zeroed.
> > > AIUI, on ARM EFI we have following call sequence:
> > >  - efi_start(),
> > >  - efi_xen_start(),
> > >  - real_start()
> > >  - ...
> > >  - el2() which zeroes BSS... ;-(((
> > >
> > > We had the same situation on x86. So, we moved BSS init just before
> > > efi_start() call and disabled later zero BSS if we are booted via EFI.
> > > Could we do the same on ARM? As I can see Jan wish to use ebmalloc on
> > > ARM too. Does it make sense for you?
> > The EFI on ARM has been designed to be standalone and disable page
> > tables, flush the cache before hand and then jump in the startup
> > beginning of the binary (as it would be done without EFI).
> >
> > The problem I can see here, is ebmalloc_mem is allocated in bss
> > rather than in init. I understand this is an optimization, to shrink
> > down the size of the binary.
> >
> > But, I am not in favor to start differing in startup code if we have
> > EFI enabled just for that.
> >
> > IHMO, anything related to the stub should be in the init section and
> > therefore will be freed when Xen has finished to boot.
> One early allocator for both platforms would be nice. And I have a feeling
> that this is the Jan's goal. However, I am not going to insist because you
> know ARM platforms better than I. So, I think that Jan should say what is
> his idea in this situation.
Out of interest, what is the reason behind putting the early allocator 
in bss rather than init?
Regards,

--
Julien Grall

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