Re: [PATCH 5/9] kern/ieee1275/init: ppc64: Add support for alignment requirements

[Stefan Berger]

On 11/10/23 09:23, Stefan Berger wrote:
> On 11/10/23 08:09, Hari Bathini wrote:
> > On 31/10/23 11:20 pm, Stefan Berger wrote:
> > > Add support for memory alignment requirements and adjust a candidate
> > > address to it before checking whether the block is large enough. This
> > > must be done in this order since the alignment adjustment can make a
> > > block smaller than what was requested.
> > >
> > > None of the current callers has memory alignment requirements but the
> > > ieee1275 loader for kernel and initrd will use it to convey them.
> > >
> > > Signed-off-by: stefan Berger <stefanb@linux.ibm.com>
> > > Cc: Hari Bathini <hbathini@linux.ibm.com>
> > > Cc: Pavithra Prakash <pavrampu@in.ibm.com>
> > > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > > Cc: Carolyn Scherrer <cpscherr@us.ibm.com>
> > > Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> > > Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
> > > ---
> > >   grub-core/kern/ieee1275/init.c        | 14 ++++++++++++++
> > >   include/grub/powerpc/ieee1275/alloc.h |  1 +
> > >   2 files changed, 15 insertions(+)
> > >
> > > diff --git a/grub-core/kern/ieee1275/init.c 
> > > b/grub-core/kern/ieee1275/init.c
> > > index a0ce9d55e..542a4c595 100644
> > > --- a/grub-core/kern/ieee1275/init.c
> > > +++ b/grub-core/kern/ieee1275/init.c
> > > @@ -506,6 +506,20 @@ regions_claim (grub_uint64_t addr, grub_uint64_t 
> > > len, grub_memory_type_t type,
> > >               }
> > >           }
> > >       }
> > > +
> > > +  /* Honor alignment restrictions on candidate addr */
> > > +  if (rcr->align)
> > > +    {
> > > +      grub_uint64_t align_addr = ALIGN_UP (addr, rcr->align);
> > > +      grub_uint64_t d = align_addr - addr;
> > > +
> > > +      if (d > len)
> > > +        return 0;
> > > +
> >
> > > +      len -= d;
> >
> > Curious.. Is 'len' here length of the region or the claim request size?
>
> len is the length of the requested size of a memory region

Actually, the above statement was not correct. It is the length of a 
free memory region that starts at addr with length len. If addr is now 
aligned-up the memory region that we can use becomes smaller by the 
number of bytes that we lost by the the alignment (align-up!), ot an 
extent that the resulting address can even become unusable ( if (d > 
len) ) if it is beyond that free region. Finally, we have to decrease 
the size of the length of the memory region by 'd'.

> > If latter, why should 'len' have any impact here??
> >
> > Thanks
> > Hari
>
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