On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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. I'm still more of a fan of this being communicated by "System RAM" being parented especially because that tells you something about how the memory is driver-managed and which mechanism might be in play. What about adding an optional /sys/firmware/memmap/X/parent attribute. This lets tooling check if it cares via that interface and lets it lookup the related infrastructure to interact with if it would do something different for virtio-mem vs dax/kmem?
