On Tue, Jul 10, 2018 at 04:25:03PM +0100, James Morse wrote:
> Hi Akashi,
>
> On 10/07/18 08:37, AKASHI Takahiro wrote:
> > On Tue, Jul 03, 2018 at 05:32:09PM +0100, James Morse wrote:
> >> On 23/06/18 03:20, AKASHI Takahiro wrote:
> >>> load_other_segments() is expected to allocate and place all the necessary
> >>> memory segments other than kernel, including initrd and device-tree
> >>> blob (and elf core header for crash).
> >>> While most of the code was borrowed from kexec-tools' counterpart,
> >>> users may not be allowed to specify dtb explicitly, instead, the dtb
> >>> presented by the original boot loader is reused.
> >>>
> >>> arch_kimage_kernel_post_load_cleanup() is responsible for freeing arm64-
> >>> specific data allocated in load_other_segments().
>
> >>> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> >>> index c38a8048ed00..7115c4f915dc 100644
> >>> --- a/arch/arm64/kernel/machine_kexec_file.c
> >>> +++ b/arch/arm64/kernel/machine_kexec_file.c
> >>
> >>> +static int setup_dtb(struct kimage *image,
> >>> + unsigned long initrd_load_addr, unsigned long initrd_len,
> >>> + char *cmdline, unsigned long cmdline_len,
> >>> + char **dtb_buf, size_t *dtb_buf_len)
> >>> +{
>
> [..]
>
> >>> + /* add bootargs */
> >>> + if (cmdline) {
> >>> + ret = fdt_setprop(buf, nodeoffset, "bootargs",
> >>> + cmdline, cmdline_len + 1);
> >>> + if (ret)
> >>> + goto out_err;
> >>> + }
> >>
> >> So (cmdline_len == 0) from user-space means keep the old cmdline. Sounds
> >> sensible. Is this documented anywhere?
>
> > To be a bit surprise, --append (and --cmdline) option belongs to arch-
> > specific options in terms of implementation. Apparently, a standard man page
> > of kexec(8) say little about it, in particular, for device-tree-based system.
> >
> > As far as arm64 is concerned, the fact is a bit complicated.
> > User space kexec accepts --append as well as --reuse-cmdline, and
> > along with yet another --dtb option, a resulting command line for new
> > kernel (or bootargs in new dtb) would look to be:
> >
> > --append=A | --reuse-cmdline | --dtb
> > | | n | y(bootargs=B)
> >
> > n n S(*1) B(*2)
> > n y S S(*3)
> > y n A A
> > y y S+A S+A(*4)
> >
> > where S: the cmdline parameters of the running system
> > (equal to bootargs in system's dtb)
>
> (I'm afraid I don't understand this table, but the text below helps...)
(Really? I believed that it was obvious.)
>
> > You are talking about case(1) above.
> >
> > Given that we can have an explicit option, --reuse-cmdline, the cmdline
> > in (1) should be NULL. Meanwhile, we specify --dtb here, which means
> > that we want to re-use system's dtb, implying that we also want to
> > reuse a cmdline parameter. (This can be arguable, though)
>
> Sounds like this is all user-space's problem!
I dare not say it's a bug :)
>
> > So I would say that we have both reasons to go for and go against.
> >
> > # Likewise,
> > # I wonder why the cmdnline would not be B or A+B, respectively,
> > in case of (3) and (4). But it's a different issue.
> >
> >> powerpc does the opposite, it deletes the bootargs in this case. Are we happy
> >> making his a per-arch thing?
> >
> > My compromise solution is:
> > a.to maintain compatibility with powerpc at system call level,
>
> This sounds like the right thing to do.
>
>
> > that is,
> > replacing bootargs in new dtb if user explicitly specifies cmdline
> > argument, otherwise nullifying bootargs,
> > b.yet maintain compatibility with arm64's kexec behavior at command line
> > interface level. If neither --append nor --dtb is not specified,
> > user space kexec will reuse the system's command line whether or not
> > --reuse-cmdline is used.
>
> (a and b aren't options this time: you're proposing to do a-and-b)
Yes.
>
> > (Do you follow me?)
>
> I think so:
> The syscall will behave the same as powerpc meaning the kernel will never re-use
> the existing cmdline in the dtb, even if that means (trying to) boot without one.
>
> If user-space wants to re-use the command line, it should read /proc/cmdline and
> feed the string back into the kernel.
Thank you for rephrasing.
>
> > So kexec and kexec_file on arm64 will still behave in exactly the same way,
> > but differently from ppc at command level for now.
>
> > The point is that, if we might want or need to change this behavior
> > (at any time in the future), we would only have to modify user space kexec.
> > Kernel semantics will never break.
> >
> > (b) requires additional small modification on kexec-tools, but kexec_file
> > support is yet to be upstreamed anyway.
>
> Makes sense!
The next version will go with this approach.
-Takahiro AKASHI
>
> Thanks,
>
> James
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