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

[dyoung@redhat.com (Dave Young)]

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:
> > > >On 01/18/16 at 07:26pm, AKASHI Takahiro wrote:
> > > >>On 01/16/2016 05:16 AM, Mark Rutland wrote:
> > > >>>On Fri, Jan 15, 2016 at 07:18:38PM +0000, Geoff Levand wrote:
> > > >>>>From: AKASHI Takahiro <takahiro.akashi@linaro.org>
> > > >>>>
> > > >>>>This patch adds arch specific descriptions about kdump usage on arm64
> > > >>>>to kdump.txt.
> > > >>>>
> > > >>>>Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> > > >>>>---
> > > >>>>  Documentation/kdump/kdump.txt | 23 ++++++++++++++++++++++-
> > > >>>>  1 file changed, 22 insertions(+), 1 deletion(-)
> > > >>>>
> > > >>>>diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
> > > >>>>index bc4bd5a..36cf978 100644
> > > >>>>--- a/Documentation/kdump/kdump.txt
> > > >>>>+++ b/Documentation/kdump/kdump.txt
> > > >>>>@@ -18,7 +18,7 @@ memory image to a dump file on the local disk, or across the network to
> > > >>>>  a remote system.
> > > >>>>
> > > >>>>  Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
> > > >>>>-s390x and arm architectures.
> > > >>>>+s390x, arm and arm64 architectures.
> > > >>>>
> > > >>>>  When the system kernel boots, it reserves a small section of memory for
> > > >>>>  the dump-capture kernel. This ensures that ongoing Direct Memory Access
> > > >>>>@@ -249,6 +249,20 @@ Dump-capture kernel config options (Arch Dependent, arm)
> > > >>>>
> > > >>>>      AUTO_ZRELADDR=y
> > > >>>>
> > > >>>>+Dump-capture kernel config options (Arch Dependent, arm64)
> > > >>>>+----------------------------------------------------------
> > > >>>>+
> > > >>>>+1) The maximum memory size on the dump-capture kernel must be limited by
> > > >>>>+   specifying:
> > > >>>>+
> > > >>>>+   mem=X[MG]
> > > >>>>+
> > > >>>>+   where X should be less than or equal to the size in "crashkernel="
> > > >>>>+   boot parameter. Kexec-tools will automatically add this.
> > > >>>
> > > >>>
> > > >>>This is extremely fragile, and will trivially fail when the kernel can
> > > >>>be loaded anywhere (see [1]).
> > > >>
> > > >>As I said before, this restriction also exists on arm, but I understand
> > > >>that recent Ard's patches break it.
> > > >>
> > > >>>We must explicitly describe the set of regions the crash kernel may use
> > > >>>(i.e. we need base and size). NAK in the absence of that.
> > > >>
> > > >>There seem to exist several approaches:
> > > >>(a) use a device-tree property, "linux,usable-memory", in addition to "reg"
> > > >>     under "memory" node
> > > >>(b) use a kernel's early parameter, "memmap=nn[@#$]ss"
> > > >>
> > > >>Power PC takes (a), while this does not work on efi-started kernel
> > > >>because dtb has no "memory" nodes under efi.
> > > >>X86 takes (b). If we take this, we will need to overwrite a weak
> > > >>early_init_dt_add_memory().
> > > >
> > > >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.

> 
> > 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.. 

> 
> 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. 

Is it possible to modify uefi memmap for kdump case?

> 
> 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?

> Thanks,
> Mark.

Thanks
Dave