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. -- Thanks, David / dhildenb
