Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP

Dan Williams
2020-05-01T21:52:16Z

On Fri, May 1, 2020 at 2:11 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 01.05.20 22:12, Dan Williams wrote:
[..]
> >>> Consider the case of EFI Special Purpose (SP) Memory that is
> >>> marked EFI Conventional Memory with the SP attribute. In that case the
> >>> firmware memory map marked it as conventional RAM, but the kernel
> >>> optionally marks it as System RAM vs Soft Reserved. The 2008 patch
> >>> simply does not consider that case. I'm not sure strict textualism
> >>> works for coding decisions.
> >>
> >> I am no expert on that matter (esp EFI). But looking at the users of
> >> firmware_map_add_early(), the single user is in arch/x86/kernel/e820.c
> >> . So the single source of /sys/firmware/memmap is (besides hotplug) e820.
> >>
> >> "'e820_table_firmware': the original firmware version passed to us by
> >> the bootloader - not modified by the kernel. ... inform the user about
> >> the firmware's notion of memory layout via /sys/firmware/memmap"
> >> (arch/x86/kernel/e820.c)
> >>
> >> How is the EFI Special Purpose (SP) Memory represented in e820?
> >> /sys/firmware/memmap is really simple: just dump in e820. No policies IIUC.
> >
> > e820 now has a Soft Reserved translation for this which means "try to
> > reserve, but treat as System RAM is ok too". It seems generically
> > useful to me that the toggle for determining whether Soft Reserved or
> > System RAM shows up /sys/firmware/memmap is a determination that
> > policy can make. The kernel need not preemptively block it.
>
> So, I think I have to clarify something here. We do have two ways to kexec
>
> 1. kexec_load(): User space (kexec-tools) crafts the memmap (e.g., using
> /sys/firmware/memmap on x86-64) and selects memory where to place the
> kexec images (e.g., using /proc/iomem)
>
> 2. kexec_file_load(): The kernel reuses the (basically) raw firmware
> memmap and selects memory where to place kexec images.
>
> We are talking about changing 1, to behave like 2 in regards to
> dax/kmem. 2. does currently not add any hotplugged memory to the
> fixed-up e820, and it should be fixed regarding hotplugged DIMMs that
> would appear in e820 after a reboot.
>
> Now, all these policy discussions are nice and fun, but I don't really
> see a good reason to (ab)use /sys/firmware/memmap for that (e.g., parent
> properties). If you want to be able to make this configurable, then
> e.g., add a way to configure this in the kernel (for example along with
> kmem) to make 1. and 2. behave the same way. Otherwise, you really only
> can change 1.

That's clearer.

>
>
> Now, let's clarify what I want regarding virtio-mem:
>
> 1. kexec should not add virtio-mem memory to the initial firmware
> memmap. The driver has to be in charge as discussed.
> 2. kexec should not place kexec images onto virtio-mem memory. That
> would end badly.
> 3. kexec should still dump virtio-mem memory via kdump.

Ok, but then seems to say to me that dax/kmem is a different type of
(driver managed) than virtio-mem and it's confusing to try to apply
the same meaning. Why not just call your type for the distinct type it
is "System RAM (virtio-mem)" and let any other driver managed memory
follow the same "System RAM ($driver)" format if it wants?