Re: [PATCH v16 11/12] powerpc: Use OF alloc and free for FDT

[Rob Herring <robh@kernel.org>]

On Thu, Feb 4, 2021 at 5:23 PM Lakshmi Ramasubramanian
<nramas@linux.microsoft.com> wrote:
>
> On 2/4/21 11:26 AM, Rob Herring wrote:
> > On Thu, Feb 4, 2021 at 10:42 AM Lakshmi Ramasubramanian
> > <nramas@linux.microsoft.com> wrote:
> >>
> >> of_alloc_and_init_fdt() and of_free_fdt() have been defined in
> >> drivers/of/kexec.c to allocate and free memory for FDT.
> >>
> >> Use of_alloc_and_init_fdt() and of_free_fdt() to allocate and
> >> initialize the FDT, and to free the FDT respectively.
> >>
> >> powerpc sets the FDT address in image_loader_data field in
> >> "struct kimage" and the memory is freed in
> >> kimage_file_post_load_cleanup().  This cleanup function uses kfree()
> >> to free the memory. But since of_alloc_and_init_fdt() uses kvmalloc()
> >> for allocation, the buffer needs to be freed using kvfree().
> >
> > You could just change the kexec core to call kvfree() instead.
>
> >
> >> Define "fdt" field in "struct kimage_arch" for powerpc to store
> >> the address of FDT, and free the memory in powerpc specific
> >> arch_kimage_file_post_load_cleanup().
> >
> > However, given all the other buffers have an explicit field in kimage
> > or kimage_arch, changing powerpc is to match arm64 is better IMO.
>
> Just to be clear:
> I'll leave this as is - free FDT buffer in powerpc's
> arch_kimage_file_post_load_cleanup() to match arm64 behavior.

Yes.

> Will not change "kexec core" to call kvfree() - doing that change would
> require changing all architectures to use kvmalloc() for
> image_loader_data allocation.

Actually, no. AIUI, kvfree() can be used whether you used kvmalloc,
vmalloc, or kmalloc for the alloc.

> >> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
> >> Suggested-by: Rob Herring <robh@kernel.org>
> >> Suggested-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
> >> ---
> >>   arch/powerpc/include/asm/kexec.h  |  2 ++
> >>   arch/powerpc/kexec/elf_64.c       | 26 ++++++++++++++++----------
> >>   arch/powerpc/kexec/file_load_64.c |  3 +++
> >>   3 files changed, 21 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/arch/powerpc/include/asm/kexec.h b/arch/powerpc/include/asm/kexec.h
> >> index 2c0be93d239a..d7d13cac4d31 100644
> >> --- a/arch/powerpc/include/asm/kexec.h
> >> +++ b/arch/powerpc/include/asm/kexec.h
> >> @@ -111,6 +111,8 @@ struct kimage_arch {
> >>          unsigned long elf_headers_mem;
> >>          unsigned long elf_headers_sz;
> >>          void *elf_headers;
> >> +
> >> +       void *fdt;
> >>   };
> >>
> >>   char *setup_kdump_cmdline(struct kimage *image, char *cmdline,
> >> diff --git a/arch/powerpc/kexec/elf_64.c b/arch/powerpc/kexec/elf_64.c
> >> index d0e459bb2f05..51d2d8eb6c1b 100644
> >> --- a/arch/powerpc/kexec/elf_64.c
> >> +++ b/arch/powerpc/kexec/elf_64.c
> >> @@ -19,6 +19,7 @@
> >>   #include <linux/kexec.h>
> >>   #include <linux/libfdt.h>
> >>   #include <linux/module.h>
> >> +#include <linux/of.h>
> >>   #include <linux/of_fdt.h>
> >>   #include <linux/slab.h>
> >>   #include <linux/types.h>
> >> @@ -32,7 +33,7 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
> >>          unsigned int fdt_size;
> >>          unsigned long kernel_load_addr;
> >>          unsigned long initrd_load_addr = 0, fdt_load_addr;
> >> -       void *fdt;
> >> +       void *fdt = NULL;
> >>          const void *slave_code;
> >>          struct elfhdr ehdr;
> >>          char *modified_cmdline = NULL;
> >> @@ -103,18 +104,12 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
> >>          }
> >>
> >>          fdt_size = fdt_totalsize(initial_boot_params) * 2;
> >> -       fdt = kmalloc(fdt_size, GFP_KERNEL);
> >> +       fdt = of_alloc_and_init_fdt(fdt_size);
> >>          if (!fdt) {
> >>                  pr_err("Not enough memory for the device tree.\n");
> >>                  ret = -ENOMEM;
> >>                  goto out;
> >>          }
> >> -       ret = fdt_open_into(initial_boot_params, fdt, fdt_size);
> >> -       if (ret < 0) {
> >> -               pr_err("Error setting up the new device tree.\n");
> >> -               ret = -EINVAL;
> >> -               goto out;
> >> -       }
> >>
> >>          ret = setup_new_fdt_ppc64(image, fdt, initrd_load_addr,
> >
> > The first thing this function does is call setup_new_fdt() which first
> > calls of_kexec_setup_new_fdt(). (Note, I really don't understand the
> > PPC code split. It looks like there's a 32-bit and 64-bit split, but
> > 32-bit looks broken to me. Nothing ever calls setup_new_fdt() except
> > setup_new_fdt_ppc64()). The arm64 version is calling
> > of_alloc_and_init_fdt() and then of_kexec_setup_new_fdt() directly.
> >
> > So we can just make of_alloc_and_init_fdt() also call
> > of_kexec_setup_new_fdt() (really, just tweak of_kexec_setup_new_fdt do
> > the alloc and copy).
> ok - will move fdt allocation into of_kexec_setup_new_fdt().
>
> I don't think the architecture needs to pick the
> > size either. It's doubtful that either one is that sensitive to the
> > amount of extra space.
> I am not clear about the above comment -
> are you saying the architectures don't need to pass FDT size to the
> alloc function?
>
> arm64 is adding command line string length and some extra space to the
> size computed from initial_boot_params for FDT Size:
>
>         buf_size = fdt_totalsize(initial_boot_params)
>                         + cmdline_len + DTB_EXTRA_SPACE;
>
> powerpc is just using twice the size computed from initial_boot_params
>
>         fdt_size = fdt_totalsize(initial_boot_params) * 2;
>
> I think it would be safe to let arm64 and powerpc pass the required FDT
> size, along with the other params to of_kexec_setup_new_fdt() - and in
> this function we allocate FDT and set it up.

It's pretty clear that someone just picked something that 'should be
enough'. The only thing I can guess for the difference is that arm
DT's tend to be a bit larger. So doubling the size would be even more
excessive. Either way, we're talking 10s kB to few 100kB. I'd go with
DTB_EXTRA_SPACE and we can bump it up if someone has problems.

Also, I would like for 'initial_boot_params' to be private ultimately,
so removing any references is helpful.

> And, for powerpc leave the remaining code in setup_new_fdt_ppc64().

Right.

Rob