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

On Sat, May 2, 2020 at 2:27 AM David Hildenbrand [off-list ref] wrote:

quoted

quoted

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?

I had the same idea but discarded it because it seemed to uglify the
add_memory() interface (passing yet another parameter only relevant for
driver managed memory). Maybe we really want a new one, because I like
that idea:

/*
 * Add special, driver-managed memory to the system as system ram.
 * The resource_name is expected to have the name format "System RAM
 * ($DRIVER)", so user space (esp. kexec-tools)" can special-case it.
 *
 * For this memory, no entries in /sys/firmware/memmap are created,
 * as this memory won't be part of the raw firmware-provided memory map
 * e.g., after a reboot. Also, the created memory resource is flagged
 * with IORESOURCE_MEM_DRIVER_MANAGED, so in-kernel users can special-
 * case this memory (e.g., not place kexec images onto it).
 */
int add_memory_driver_managed(int nid, u64 start, u64 size,
                              const char *resource_name);


If we'd ever have to special case it even more in the kernel, we could
allow to specify further resource flags. While passing the driver name
instead of the resource_name would be an option, this way we don't have
to hand craft new resource strings for added memory resources.

Thoughts?

Looks useful to me and simplifies walking /proc/iomem. I personally
like the safety of the string just being the $driver component of the
name, but I won't lose sleep if the interface stays freeform like you
propose.