[PATCH v24 5/9] arm64: kdump: add kdump support

[panand@redhat.com (Pratyush Anand)]

On 23/08/2016:09:38:16 AM, AKASHI Takahiro wrote:
> On Mon, Aug 22, 2016 at 02:47:30PM +0100, James Morse wrote:
> > On 22/08/16 02:29, AKASHI Takahiro wrote:
> > > On Fri, Aug 19, 2016 at 04:52:17PM +0530, Pratyush Anand wrote:
> > >> It will help kexec-tools to prevent copying  of any unnecessary data. I
> > >> think, then you also need to change phys_offset calculation in kexec-tools. That
> > >> should be start of either of first "reserved" or "System RAM" block.
> > > 
> > > Good point, but I'm not sure this is always true.
> > 
> > > Is there any system whose ACPI memory is *not* part of DRAM
> > 
> > From the spec, it looks like this is allowed.
> > 
> > What do you mean by 'DRAM'? Any ACPI region will be in the UEFI memory map, so
> > the question is what is its type and memory attributes?
> 
> Yes.
> 
> > The UEFI spec[0] says ACPI regions can have a type of EfiACPIReclaimMemory or
> > EfiACPIMemoryNVS, the memory attributes aren't specified, so are chosen by the
> > firmware.
> > 
> > It is possible these regions have to be mapped non-cacheable, page 40 has a
> > couple of:
> > > If no information about the table location exists in the UEFI memory map or
> > ACPI memory
> > > descriptors, the table is assumed to be non-cached.
> > 
> > reserve_regions() in drivers/firmware/efi/arm-init.c will add any entry in the
> > memory map that has a 'WB' attribute to the memblock.memory list (via
> > early_init_dt_add_memory_arch()), it will also mark as no-map regions that have
> > this attribute and aren't in the is_reserve_region() list.
> > 
> > If these ACPI regions have the 'WB' attribute, we add them as memory and mark
> > them nomap. These show up as either a hole, or 'reserved' in /proc/iomem.
> > If they don't have the 'WB' attribute, then then they are left out of memblock
> > and aren't part of DRAM, I don't think these will show up in /proc/iomem at all.
> 
> Let's say,
> 0x1000-0x1fff: reserved (SRAM for UEFI, WB)
> 0x80000000-0xffffffff: System RAM (DRAM)

May be slightly more complicated:
0x80000000-0x80001fff: System RAM (DRAM) for UEFI, WB
0x80002000-0xffffffff: System RAM (DRAM)

Kernel will have phys_offset 0x80000000, however kexec-tools will calculate it
as 0x80002000.

> 
> If, as Pratyush suggested, "reserved" resources are added to phys_offset
> calculation, the kernel linear mapping area starts at PAGE_OFFSET, but
> there is no actual mapping around PAGE_OFFSET.
> It won't hurt anything, but looks funny.
> So we'd better not include "reserved" in phys_offset calculation anyway.
>         -> Pratyush

My only concern is that, then we will have different values of phys_offset in
kernel and kexec-tools, which might lead to further confusion.

~Pratyush