On Fri, May 1, 2020 at 10:21 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 01.05.20 18:56, Dan Williams wrote: > > 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" > > I was mentioning somewhere in this thread that "System RAM" inside a > hierarchy (like dax/kmem) will already be basically ignored by > kexec-tools. So, placing it inside a hierarchy already makes it look > special already. > > But after all, as we have to change kexec-tools either way, we can > directly go ahead and flag it properly as special (in case there will > ever be other cases where we could no longer distinguish it). > > > being parented especially because that tells you something about how > > the memory is driver-managed and which mechanism might be in play. > > The could be communicated to some degree via the resource hierarchy. > > E.g., > > [root@localhost ~]# cat /proc/iomem > ... > 140000000-33fffffff : Persistent Memory > 140000000-1481fffff : namespace0.0 > 150000000-33fffffff : dax0.0 > 150000000-33fffffff : System RAM (driver managed) > > vs. > > :/# cat /proc/iomem > [...] > 140000000-333ffffff : virtio-mem (virtio0) > 140000000-147ffffff : System RAM (driver managed) > 148000000-14fffffff : System RAM (driver managed) > 150000000-157ffffff : System RAM (driver managed) > > Good enough for my taste. > > > What about adding an optional /sys/firmware/memmap/X/parent attribute. > > I really don't want any firmware memmap entries for something that is > not part of the firmware provided memmap. In addition, > /sys/firmware/memmap/ is still a fairly x86_64 specific thing. Only mips > and two arm configs enable it at all. > > So, IMHO, /sys/firmware/memmap/ is definitely not the way to go. I think that's a policy decision and policy decisions do not belong in the kernel. Give the tooling the opportunity to decide whether System RAM stays that way over a kexec. The parenthetical reference otherwise looks out of place to me in the /proc/iomem output. What makes it "driver managed" is how the kernel handles it, not how the kernel names it.
