On Fri, May 1, 2020 at 10:51 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 01.05.20 19:45, David Hildenbrand wrote: > > On 01.05.20 19:39, Dan Williams wrote: > >> 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. > > > > At least, virtio-mem is different. It really *has to be handled* by the > > driver. This is not a policy. It's how it works. ...but that's not necessarily how dax/kmem works. > > > > Oh, and I don't see why "System RAM (driver managed)" would hinder any > policy in user case to still do what it thinks is the right thing to do > (e.g., for dax). > > "System RAM (driver managed)" would mean: Memory is not part of the raw > firmware memmap. It was detected and added by a driver. Handle with > care, this is special. Oh, no, I was more reacting to your, "don't update /sys/firmware/memmap for the (driver managed) range" choice as being a policy decision. It otherwise feels to me "System RAM (driver managed)" adds confusion for casual users of /proc/iomem and for clued in tools they have the parent association to decide policy.
