[PATCH 18/19] arm64: kdump: update a kernel doc

[dyoung@redhat.com (Dave Young)]

On 01/19/16 at 02:01pm, Mark Rutland wrote:
> On Tue, Jan 19, 2016 at 09:45:53PM +0800, Dave Young wrote:
> > On 01/19/16 at 12:51pm, Mark Rutland wrote:
> > > On Tue, Jan 19, 2016 at 08:28:48PM +0800, Dave Young wrote:
> > > > On 01/19/16 at 02:35pm, AKASHI Takahiro wrote:
> > > > > On 01/19/2016 10:43 AM, Dave Young wrote:
> > > > > >X86 takes another way in latest kexec-tools and kexec_file_load, that is
> > > > > >recreating E820 table and pass it to kexec/kdump kernel, if the entries
> > > > > >are over E820 limitation then turn to use setup_data list for remain
> > > > > >entries.
> > > > > 
> > > > > Thanks. I will visit x86 code again.
> > > > > 
> > > > > >I think it is X86 specific. Personally I think device tree property is
> > > > > >better.
> > > > > 
> > > > > Do you think so?
> > > > 
> > > > I'm not sure it is the best way. For X86 we run into problem with
> > > > memmap= design, one example is pci domain X (X>1) need the pci memory
> > > > ranges being passed to kdump kernel. When we passed reserved ranges in /proc/iomem
> > > > to 2nd kernel we find that cmdline[] array is not big enough.
> > > 
> > > I'm not sure how PCI ranges relate to the memory map used for normal
> > > memory (i.e. RAM), though I'm probably missing some caveat with the way
> > > ACPI and UEFI describe PCI. Why does memmap= affect PCI memory?
> > 
> > Here is the old patch which was rejected in kexec-tools:
> > http://lists.infradead.org/pipermail/kexec/2013-February/007924.html
> > 
> > > 
> > > If the kernel got the rest of its system topology from DT, the PCI
> > > regions would be described there.
> > 
> > Yes, if kdump kernel use same DT as 1st kernel.
> 
> Other than for testing purposes, I don't see why you'd pass the kdump
> kernel a DTB inconsistent with that the 1st kernel was passsed (other
> than some proerties under /chosen).
> 
> We added /sys/firmware/fdt specifically to allow the kexec tools to get
> the exact DTB the first kernel used. There's no reason for tools to have
> to make something up.

Agreed, but kexec-tools has an option to pass in any dtb files. Who knows
how one will use it unless dropping the option and use /sys/firmware/fdt
unconditionally. 

If we choose to implement kexec_file_load only in kernel, the interfaces
provided are kernel, initrd and cmdline. We can always use same dtb.
 
> 
> > > > Do you think for arm64 only usable memory is necessary to let kdump kernel
> > > > know? I'm curious about how arm64 kernel get all memory layout from boot loader,
> > > > via UEFI memmap?
> > > 
> > > When booted via EFI, we use the EFI memory map. The EFI stub handles
> > > acquring the relevant information and passing that to the first kernel
> > > in the DTB (see Documentation/arm/uefi.txt).
> > 
> > Ok, thanks for the pointer. So in dt we are just have uefi memmap infomation
> > instead of memory nodes details.. 
> 
> When booted via EFI, yes.
> 
> For NUMA topology in !ACPI kernels, we might need to also retain and
> parse memory nodes, but only for toplogy information. The kernel would
> still only use memory as described by the EFI memory map.
> 
> There's a horrible edge case I've spotted if performing a chain of
> cross-endian kexecs: LE -> BE -> LE, as the BE kernel would have to
> respect the EFI memory map so as to avoid corrupting it for the
> subsequent LE kernel. Other than this I believe everything should just
> work.

Firmware do not know kernel endianniess, kernel should respect firmware
maps and adapt to it, it sounds like a generic issue not specfic to kexec.

> 
> > > A kexec'd kernel should simply inherit that. So long as the DTB and/or
> > > UEFI tables in memory are the same, it would be the same as a cold boot.
> > 
> > For kexec all memory ranges are same, for kdump we need use original reserved
> > range with crashkernel= as usable memory and all other orignal usable ranges
> > are not usable anymore. 
> 
> Sure. This is what I believe we should expose with an additional
> property under /chosen, while keeping everything else pristine.
> 
> The crash kernel can then limit itself to that region, while it would
> have the information of the full memory map (which it could log and/or
> use to drive other dumping).

In this way kernel should be aware it is a kdump booting, it is doable though
I feel it is better for kdump kernel in a black box with infomations it
can use just like the 1st kernel. Things here is where we choose to cook
the memory infomation in boot loader or in kernel itself.

> 
> > Is it possible to modify uefi memmap for kdump case?
> 
> Technically it would be possible, however I don't think it's necessary,
> and I think it would be disadvantageous to do so.
> 
> Describing the range(s) the crash kernel can use in separate properties
> under /chosen has a number of advantages.

Ok, I got the points. We have a is_kdump_kernel() by checking if there is
elfcorehdr_addr kernel cmdline. This is mainly for some drivers which
do not work well in kdump kernel some uncertain reasons. But ideally I
think kernel should handle things just like in 1st kernel and avoid to use
it. 

> 
> > > In the !EFI case, we use the memory nodes in the DTB. Only in this case
> > > could usable-memory properties in memory nodes make sense. I'd prefer a
> > > uniform property under /chosen for both cases.
> > 
> > We stil use same DTB, need to modify the DT and update the usable and unusable
> > nodes for kdump?
> 
> We'd have a (slightly) modified DTB that contained additional properties
> describing the range(s) reserved for use by the crash kernel.
> 
> Other than those properties under /chosen (e.g. the command line, initrd
> pointers if any), it would be the original DTB.
> 
> Thanks,
> Mark.

Thanks
Dave