On 01.05.20 00:24, Andrew Morton wrote: > On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote: > >>> >>> Why does the firmware map support hotplug entries? >> >> I assume: >> >> The firmware memmap was added primarily for x86-64 kexec (and still, is >> mostly used on x86-64 only IIRC). There, we had ACPI hotplug. When DIMMs >> get hotplugged on real HW, they get added to e820. Same applies to >> memory added via HyperV balloon (unless memory is unplugged via >> ballooning and you reboot ... the the e820 is changed as well). I assume >> we wanted to be able to reflect that, to make kexec look like a real reboot. >> >> This worked for a while. Then came dax/kmem. Now comes virtio-mem. >> >> >> But I assume only Andrew can enlighten us. >> >> @Andrew, any guidance here? Should we really add all memory to the >> firmware memmap, even if this contradicts with the existing >> documentation? (especially, if the actual firmware memmap will *not* >> contain that memory after a reboot) > > For some reason that patch is misattributed - it was authored by > Shaohui Zheng <shaohui.zheng@xxxxxxxxx>, who hasn't been heard from in > a decade. I looked through the email discussion from that time and I'm > not seeing anything useful. But I wasn't able to locate Dave Hansen's > review comments. Okay, thanks for checking. I think the documentation from 2008 is pretty clear what has to be done here. I will add some of these details to the patch description. Also, now that I know that esp. kexec-tools already don't consider dax/kmem memory properly (memory will not get dumped via kdump) and won't really suffer from a name change in /proc/iomem, I will go back to the MHP_DRIVER_MANAGED approach and 1. Don't create firmware memmap entries 2. Name the resource "System RAM (driver managed)" 3. Flag the resource via something like IORESOURCE_MEM_DRIVER_MANAGED. This way, kernel users and user space can figure out that this memory has different semantics and handle it accordingly - I think that was what Eric was asking for. Of course, open for suggestions. -- Thanks, David / dhildenb
