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

[Pingfan Liu]

----- Original Message -----
> From: "Dave Young" <dyoung@xxxxxxxxxx>
> To: "AKASHI Takahiro" <takahiro.akashi@xxxxxxxxxx>, "Philipp Rudo" <prudo@xxxxxxxxxxxxx>, "catalin marinas"
> <catalin.marinas@xxxxxxx>, "will deacon" <will.deacon@xxxxxxx>, dhowells@xxxxxxxxxx, vgoyal@xxxxxxxxxx,
> herbert@xxxxxxxxxxxxxxxxxxx, davem@xxxxxxxxxxxxx, bhe@xxxxxxxxxx, arnd@xxxxxxxx, schwidefsky@xxxxxxxxxx, "heiko
> carstens" <heiko.carstens@xxxxxxxxxx>, "ard biesheuvel" <ard.biesheuvel@xxxxxxxxxx>, "james morse"
> <james.morse@xxxxxxx>, bhsharma@xxxxxxxxxx, kexec@xxxxxxxxxxxxxxxxxxx, linux-arm-kernel@xxxxxxxxxxxxxxxxxxx,
> linux-kernel@xxxxxxxxxxxxxxx, "piliu@xxxxxxxxxx Thiago Jung Bauermann" <bauerman@xxxxxxxxxxxxxxxxxx>
> Sent: Thursday, August 9, 2018 11:34:16 AM
> Subject: Re: [PATCH v13 03/16] s390, kexec_file: drop arch_kexec_mem_walk()
> 
> 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?
> 

ppc64 uses a few KB starting from 0 for exception handler.

> 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
> 
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