On 01/19/16 at 02:05pm, Mark Rutland wrote: > On Tue, Jan 19, 2016 at 09:52:33PM +0800, Dave Young wrote: > > On 01/19/16 at 12:17pm, Mark Rutland wrote: > > > On Tue, Jan 19, 2016 at 09:43:32AM +0800, 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 at linaro.org> > > > > > >> > > > > > >>This patch adds arch specific descriptions about kdump usage on arm64 > > > > > >>to kdump.txt. > > > > > >> > > > > > >>Signed-off-by: AKASHI Takahiro <takahiro.akashi at 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. > > > > > > This would imply modifying the EFI memory map or the memory nodes, which > > > I'm not keen on. > > > > > > I would prefer that they are left _pristine_, and we describe the > > > restriction on the kdump kernel with additional properties under > > > /chosen. > > > > > > That leaves us with more useful information about the environment of the > > > first kernel, is simpler for userspace (it's resilient to updates to the > > > UEFI memory map spec, for example), and is simple for the crash kernel. > > > > In theory kexec as boot loader should prepare correct efi memmap and pass > > to kernel, but as you said yes it will increase complexity. We need banlance > > them. > > I'd argue that the "correct efi memmap" is what we were given by the > firmware initially -- none of that information is any less true. In X86 boot loader will cook a E820 map for kernel use, there's no such needs in arm so maybe it is acceptable to use same memmap to avoid modifying it only for kdump. I think I will not insist though I like more about doing something in bootloader instead of in kernel. > > For kdump all we need to ensure is that the kdump kernel only uses the > memory that was specially reserved for it by the first kernel. The > simplest way of doing that is to tell the kdump kernel which specific > region(s) of memory were reserved for it, leaving the EFI memory map > alone. Yes, agreed that it is simpler. > > Thanks, > Mark. Thanks Dave
