Re: [PATCH v10 08/14] arm64: kexec_file: load initrd and device-tree

[James Morse]

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...)


> 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!


> 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)


> (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.


> 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!


Thanks,

James