Re: [PATCH v13 03/16] s390, kexec_file: drop arch_kexec_mem_walk()

[Dave Young]

Add more cc. Pingfan Liu confirmed ppc does not use 0 as valid address,
if so it should be safe.

Pingfan, can you add more words?

On 08/06/18 at 01:50pm, Dave Young wrote:
> Add Thiago in cc so that he can review from powerpc point of view. 
> 
> On 08/02/18 at 09:01am, AKASHI Takahiro wrote:
> > Hi,
> > 
> > On Wed, Aug 01, 2018 at 10:29:51AM +0200, Philipp Rudo wrote:
> > > Hey Akashi,
> > > 
> > > I kept thinking about this patch and remembered why I didn't made the change
> > > you are suggesting now.
> > 
> > Hmm.
> > 
> > > The problem is when you only check for kbuf->mem you are excluding address 0,
> > > which might be a valid address to load the kernel to. On s390 this is actually
> > > done when the kernel is not loaded via a boot loader. For kexec_file however,
> > > we cut off the first few kB of the image and jump directly to 'startup'. So
> > > checking for !0 does not cause a problem here.
> > 
> > Yeah, as Dave(RedHat) described, all the current kexec-capable architectures,
> > except arm64, implicitly initialize kbuf.mem to zero with "kbuf = { ... }"
> > initializer. So surely my change would not break anything on the existing code.
> > On the other hand, I also see your concern here.
> > 
> > > Anyway, the long term safer solution would be something like
> > > 
> > > #define KEXEC_BUF_MEM_UNKNOWN (-1UL)
> > > 
> > > for architectures to tell common code to search a fitting mem hole.
> > 
> > This would require the existing code on every arch to be modified, which
> > I think should be avoided if possible. Instead,
> > we'd better define in linux/kexec.h:
> >   #ifndef KEXEC_BUF_MEM_UNKNOWN
> >   #define KEXEC_BUF_MEM_UNKNOWN 0
> >   #endif
> > and then check for kbuf in kexec_locate_mem_hole():
> >   if (kbuf->mem != KEXEC_BUF_MEM_UNKNOWN)
> >         return 0;
> >   ...
> > 
> > This way if some arch wants to treat *zero* as a valid address, it can
> > redefine this macro in arch/asm/kexec.h.
> 
> I think this way works if powerpc is safe for using zero as the unknown
> address in this case.  Thiago, can you provide some review?
> 
> Philipp, thanks for catching the problem!
> 
> > 
> > Thanks,
> > -Takahiro AKASHI
> > 
> > > 
> > > Back then I didn't do the change because I had the other workaround, which
> > > didn't require a common code change. But when you are touching the code now it
> > > is worth thinking about it.
> > > 
> > > Just wanted to let you know
> > > Philipp
> > > 
> > > 
> > > On Wed,  1 Aug 2018 16:58:07 +0900
> > > AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx> wrote:
> > > 
> > > > Since s390 already knows where to locate buffers, calling
> > > > arch_kexec_mem_walk() has no sense. So we can just drop it as kbuf->mem
> > > > indicates this while all other architectures sets it to 0 initially.
> > > > 
> > > > This change is a preparatory work for the next patch, where all the
> > > > variant memory walks, either on system resource or memblock, will be
> > > > put in one common place so that it will satisfy all the architectures'
> > > > need.
> > > > 
> > > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > > > Reviewed-by: Philipp Rudo <prudo@xxxxxxxxxxxxx>
> > > > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > > > Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> > > > Cc: Dave Young <dyoung@xxxxxxxxxx>
> > > > Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> > > > Cc: Baoquan He <bhe@xxxxxxxxxx>
> > > > ---
> > > >  arch/s390/kernel/machine_kexec_file.c | 10 ----------
> > > >  kernel/kexec_file.c                   |  4 ++++
> > > >  2 files changed, 4 insertions(+), 10 deletions(-)
> > > > 
> > > > diff --git a/arch/s390/kernel/machine_kexec_file.c b/arch/s390/kernel/machine_kexec_file.c
> > > > index f413f57f8d20..32023b4f9dc0 100644
> > > > --- a/arch/s390/kernel/machine_kexec_file.c
> > > > +++ b/arch/s390/kernel/machine_kexec_file.c
> > > > @@ -134,16 +134,6 @@ int kexec_file_add_initrd(struct kimage *image, struct s390_load_data *data,
> > > >  	return ret;
> > > >  }
> > > >  
> > > > -/*
> > > > - * The kernel is loaded to a fixed location. Turn off kexec_locate_mem_hole
> > > > - * and provide kbuf->mem by hand.
> > > > - */
> > > > -int arch_kexec_walk_mem(struct kexec_buf *kbuf,
> > > > -			int (*func)(struct resource *, void *))
> > > > -{
> > > > -	return 1;
> > > > -}
> > > > -
> > > >  int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
> > > >  				     Elf_Shdr *section,
> > > >  				     const Elf_Shdr *relsec,
> > > > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > > > index 63c7ce1c0c3e..bf39df5e5bb9 100644
> > > > --- a/kernel/kexec_file.c
> > > > +++ b/kernel/kexec_file.c
> > > > @@ -534,6 +534,10 @@ int kexec_locate_mem_hole(struct kexec_buf *kbuf)
> > > >  {
> > > >  	int ret;
> > > >  
> > > > +	/* Arch knows where to place */
> > > > +	if (kbuf->mem)
> > > > +		return 0;
> > > > +
> > > >  	ret = arch_kexec_walk_mem(kbuf, locate_mem_hole_callback);
> > > >  
> > > >  	return ret == 1 ? 0 : -EADDRNOTAVAIL;
> > > 
> > 
> > _______________________________________________
> > kexec mailing list
> > kexec@xxxxxxxxxxxxxxxxxxx
> > http://lists.infradead.org/mailman/listinfo/kexec
> 
> Thanks
> Dave