On 16 September 2016 at 17:04, James Morse <james.morse at arm.com> wrote: > (Cc: Ard), > > Mark, Ard, how does/will reserved-memory work on an APCI only system? > It works by accident, at the moment. We used to ignore both /memreserve/s and the /reserved-memory node, but due to some unrelated refactoring, we ended up honouring the reserved-memory node when booting via UEFI I proposed some patches a while ago to at least check the reservations, given that UEFI itself is unaware of them and may end up occupying a region that should have been reserved. http://article.gmane.org/gmane.linux.kernel.efi/6464 > > On 07/09/16 05:29, AKASHI Takahiro wrote: >> v26-specific note: After a comment from Rob[0], an idea of adding >> "linux,usable-memory-range" was dropped. Instead, an existing >> "reserved-memory" node will be used to limit usable memory ranges >> on crash dump kernel. >> This works not only on UEFI/ACPI systems but also on DT-only systems, >> but if he really insists on using DT-specific "usable-memory" property, >> I will post additional patches for kexec-tools. Those would be >> redundant, though. >> Even in that case, the kernel will not have to be changed. > > Some narrative on how the old memory ranges get reserved, as there is no longer > any code in the series doing this, (which is pretty neat!): > > kexec-tools parses the list of memory ranges in /proc/iomem, and adds a node to > the /reserved-memory for System RAM ranges that don't cover the crash kernel. > Decompiling the crash-kernel DT from Seattle, it looks roughly like this: > > reserved-memory { > ranges; > #size-cells = <0x2>; > #address-cells = <0x2>; > > crash_dump at 83ffe50000 { > no-map; > reg = <0x83 0xffe50000 0x0 0x1b0000>; > }; > > [ ... ] > }; > > > 'no-map' means its doing the same thing to memblock as > 'linux,usable-memory-range' did in earlier versions, > early_init_dt_reserve_memory_arch() takes no-map to mean memblock_remove(). > We trigger the removing via early_init_fdt_scan_reserved_mem() in > arch/arm64/mm/init.c. This happens later than before, but its before the > crashkernel and cma ranges get reserved. > > One difference I can see is that before we avoided memblock_remove()ing ranges > that were also in memblock.nomap. This was to avoid the ACPI tables getting > mapped as device memory by mistake, this is fixed by [1]. Now these ranges are > published in /proc/iomem as 'reserved' and won't get covered by a > reserved-memory node, and so we don't need to check memblock.nomap when > memblock_remove()ing. > > > The only odd thing I can see is for a (mythical?) pure-ACPI system. The EFI stub > will create a DT with a chosen node containing pointers to the memory map and > the efi command line. Now such as system may also grow a /reserved-memory node > after kdump. I don't think this is a problem, but it may not match how an > acpi-only system reserves memory. (how does that work?) > > >> [1] "arm64: mark reserved memblock regions explicitly in iomem" >> http://lists.infradead.org/pipermail/linux-arm-kernel/2016-August/450433.html > > This is queued in Will's arm64/for-next/core, > >> [2] "efi: arm64: treat regions with WT/WC set but WB cleared as memory" >> http://lists.infradead.org/pipermail/linux-arm-kernel/2016-August/451491.html > > This is queued in tip, but I can't see why kdump depends on it. It only has an > effect if the uefi memory map has !WB regions that linux needs to use. > > > > Thanks, > > James >
