Date:   Mon, 21 May 2018 18:46:59 +0900
From:   AKASHI Takahiro <takahiro.akashi@...aro.org>
To:     James Morse <james.morse@....com>
Cc:     catalin.marinas@....com, will.deacon@....com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, dyoung@...hat.com, bhe@...hat.com,
        arnd@...db.de, ard.biesheuvel@...aro.org, bhsharma@...hat.com,
        kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 07/11] arm64: kexec_file: add crash dump support

James,

On Fri, May 18, 2018 at 05:00:55PM +0100, James Morse wrote:
> Hi Akashi,
> 
> On 18/05/18 11:39, AKASHI Takahiro wrote:
> > On Tue, May 15, 2018 at 06:11:15PM +0100, James Morse wrote:
> >> On 25/04/18 07:26, AKASHI Takahiro wrote:
> >>> Enabling crash dump (kdump) includes
> >>> * prepare contents of ELF header of a core dump file, /proc/vmcore,
> >>>   using crash_prepare_elf64_headers(), and
> >>> * add two device tree properties, "linux,usable-memory-range" and
> >>>   "linux,elfcorehdr", which represent repsectively a memory range
> 
> >>> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> >>> index 37c0a9dc2e47..ec674f4d267c 100644
> >>> --- a/arch/arm64/kernel/machine_kexec_file.c
> >>> +++ b/arch/arm64/kernel/machine_kexec_file.c
> 
> >>> +static void fill_property(void *buf, u64 val64, int cells)
> >>> +{
> >>> +	u32 val32;
> >>> +
> >>> +	if (cells == 1) {
> >>> +		val32 = cpu_to_fdt32((u32)val64);
> >>> +		memcpy(buf, &val32, sizeof(val32));
> >>> +	} else {
> >>
> >>> +		memset(buf, 0, cells * sizeof(u32) - sizeof(u64));
> >>> +		buf += cells * sizeof(u32) - sizeof(u64);
> >>
> >> Is this trying to clear the 'top' cells and shuffle the pointer to point at the
> >> 'bottom' 2? I'm pretty sure this isn't endian safe.
> >>
> >> Do we really expect a system to have #address-cells > 2?
> > 
> > I don't know, but just for safety.
> 
> Okay, so this is aiming to be a cover-all-cases library function.
> 
> 
> >>> +		val64 = cpu_to_fdt64(val64);
> >>> +		memcpy(buf, &val64, sizeof(val64));
> >>> +	}
> >>> +}
> >>> +
> >>> +static int fdt_setprop_range(void *fdt, int nodeoffset, const char *name,
> >>> +				unsigned long addr, unsigned long size)
> >>
> >> (the device-tree spec describes a 'ranges' property, which had me confused. This
> >> is encoding a prop-encoded-array)
> > 
> > Should we rename it to, say, fdt_setprop_reg()?
> 
> Sure, but I'd really like this code to come from libfdt. I'm hoping for some
> temporary workaround, lets see what the DT folk say.

OK, I will follow Rob's suggestion.

> >>> +	if (!buf)
> >>> +		return -ENOMEM;
> >>> +
> >>> +	fill_property(prop, addr, __dt_root_addr_cells);
> >>> +	prop += __dt_root_addr_cells * sizeof(u32);
> >>> +
> >>> +	fill_property(prop, size, __dt_root_size_cells);
> >>> +
> >>> +	result = fdt_setprop(fdt, nodeoffset, name, buf, buf_size);
> >>> +
> >>> +	vfree(buf);
> >>> +
> >>> +	return result;
> >>> +}
> >>
> >> Doesn't this stuff belong in libfdt? I guess there is no 'add array element' api
> >> because this the first time we've wanted to create a node with more than
> >> key=fixed-size-value.
> >>
> >> I don't think this belongs in arch C code. Do we have a plan for getting libfdt
> >> to support encoding prop-arrays? Can we put it somewhere anyone else duplicating
> >> this will find it, until we can (re)move it?
> > 
> > I will temporarily move all fdt-related stuff to a separate file, but
> > 
> >> I have no idea how that happens... it looks like the devicetree list is the
> >> place to ask.
> > 
> > should we always sync with the original dtc/libfdt repository?
> 
> I thought so, libfdt is one of those external libraries that the kernel
> consumes, like acpica. For acpica at least the rule is changes go upstream, then
> get sync'd back.

Same above.

> >>>  static int setup_dtb(struct kimage *image,
> >>>  		unsigned long initrd_load_addr, unsigned long initrd_len,
> >>>  		char *cmdline, unsigned long cmdline_len,
> >>> @@ -88,10 +165,26 @@ static int setup_dtb(struct kimage *image,
> >>>  	int range_len;
> >>>  	int ret;
> >>>  
> >>> +	/* check ranges against root's #address-cells and #size-cells */
> >>> +	if (image->type == KEXEC_TYPE_CRASH &&
> >>> +		(!cells_size_fitted(image->arch.elf_load_addr,
> >>> +				image->arch.elf_headers_sz) ||
> >>> +		 !cells_size_fitted(crashk_res.start,
> >>> +				crashk_res.end - crashk_res.start + 1))) {
> >>> +		pr_err("Crash memory region doesn't fit into DT's root cell sizes.\n");
> >>> +		ret = -EINVAL;
> >>> +		goto out_err;
> >>> +	}
> >>
> >> To check I've understood this properly: This can happen if the firmware provided
> >> a DTB with 32bit address/size cells, but at least some of the memory requires 64
> >> bit address/size cells. This could only happen on a UEFI system where the
> >> firmware-DTB doesn't describe memory. ACPI-only systems would have the EFIstub DT.
> > 
> > Probably, yes. I assumed the case where #address-cells and #size-cells
> > were just missing in fdt.
> 
> Ah, that's another one. I just wanted to check we could boot on a system where
> this can happen.
> 
> 
> >>>  	/* duplicate dt blob */
> >>>  	buf_size = fdt_totalsize(initial_boot_params);
> >>>  	range_len = (__dt_root_addr_cells + __dt_root_size_cells) * sizeof(u32);
> >>>  
> >>> +	if (image->type == KEXEC_TYPE_CRASH)
> >>> +		buf_size += fdt_prop_len("linux,elfcorehdr", range_len)
> >>> +				+ fdt_prop_len("linux,usable-memory-range",
> >>> +								range_len);
> 
> >                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> [...]
> 
> >> Don't you need to add "linux,usable-memory-range" to the buf_size estimate?
> > 
> > I think the code exists. See above.
> 
> Sorry, turns out I can't read!
> 
> 
> >>> +		if (ret)
> >>> +			goto out_err;
> >>> +	}
> >>
> >>> @@ -148,17 +258,109 @@ static int setup_dtb(struct kimage *image,
> >>
> >>> +static struct crash_mem *get_crash_memory_ranges(void)
> >>> +{
> >>> +	unsigned int nr_ranges;
> >>> +	struct crash_mem *cmem;
> >>> +
> >>> +	nr_ranges = 1; /* for exclusion of crashkernel region */
> >>> +	walk_system_ram_res(0, -1, &nr_ranges, get_nr_ranges_callback);
> >>> +
> >>> +	cmem = vmalloc(sizeof(struct crash_mem) +
> >>> +			sizeof(struct crash_mem_range) * nr_ranges);
> >>> +	if (!cmem)
> >>> +		return NULL;
> >>> +
> >>> +	cmem->max_nr_ranges = nr_ranges;
> >>> +	cmem->nr_ranges = 0;
> >>> +	walk_system_ram_res(0, -1, cmem, add_mem_range_callback);
> >>> +
> >>> +	/* Exclude crashkernel region */
> >>> +	if (crash_exclude_mem_range(cmem, crashk_res.start, crashk_res.end)) {
> >>> +		vfree(cmem);
> >>> +		return NULL;
> >>> +	}
> >>> +
> >>> +	return cmem;
> >>> +}
> >>
> >> Could this function be included in prepare_elf_headers() so that the alloc() and
> >> free() occur together.
> > 
> > Or aiming that arm64 and x86 have similar-look code?
> 
> What's the advantage in things looking the same? If they are the same, it
> probably shouldn't be in per-arch code. Otherwise it should be as simple as
> possible, otherwise we can't spot the bugs/leaks.
> 
> But I think walking memblock here will remove all 'looks the same' properties here.

OK, I will unfold the function in prepare_elf_headers().

> 
> >>> +static int prepare_elf_headers(void **addr, unsigned long *sz)
> >>> +{
> >>> +	struct crash_mem *cmem;
> >>> +	int ret = 0;
> >>> +
> >>> +	cmem = get_crash_memory_ranges();
> >>> +	if (!cmem)
> >>> +		return -ENOMEM;
> >>> +
> >>> +	ret =  crash_prepare_elf64_headers(cmem, true, addr, sz);
> >>> +
> >>> +	vfree(cmem);
> >>
> >>> +	return ret;
> >>> +}
> >>
> >> All this is moving memory-range information from core-code's
> >> walk_system_ram_res() into core-code's struct crash_mem, and excluding
> >> crashk_res, which again is accessible to the core code.
> >>
> >> It looks like this is duplicated in arch/x86 and arch/arm64 because arm64
> >> doesn't have a second 'crashk_low_res' region, and always wants elf64, instead
> >> of when IS_ENABLED(CONFIG_X86_64).
> >> If we can abstract just those two, more of this could be moved to core code
> >> where powerpc can make use of it if they want to support kdump with
> >> kexec_file_load().
> >>
> >> But, its getting late for cross-architecture dependencies, lets put that on the
> >> for-later list. (assuming there isn't a powerpc-kdump series out there adding a
> >> third copy of this)
> > 
> > Sure. X86 code has so many exceptional lines in the code :)
> 
> They also pass the e820 'usable-memory' map on the cmdline...

Well, according to Dave(RedHat)'s past comment, this type of kernel
parameters are in a old style, and x86 now has a dedicated memory region
passed for this sake.

Thanks,
-Takahiro AKASHI

> 
> Thanks,
> 
> James

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