On Wed, Jan 08, 2025 at 07:04:23AM -0500, Michael S. Tsirkin wrote:

On Wed, Dec 04, 2024 at 01:54:31PM +0100, David Hildenbrand wrote:

quoted

The only "different than everything else" thing about virtio-mem on s390
is kdump: The crash (2nd) kernel allocates+prepares the elfcore hdr
during fs_init()->vmcore_init()->elfcorehdr_alloc(). Consequently, the
kdump kernel must detect memory ranges of the crashed kernel to
include via PT_LOAD in the vmcore.

On other architectures, all RAM regions (boot + hotplugged) can easily be
observed on the old (to crash) kernel (e.g., using /proc/iomem) to create
the elfcore hdr.

On s390, information about "ordinary" memory (heh, "storage") can be
obtained by querying the hypervisor/ultravisor via SCLP/diag260, and
that information is stored early during boot in the "physmem" memblock
data structure.

But virtio-mem memory is always detected by as device driver, which is
usually build as a module. So in the crash kernel, this memory can only be
properly detected once the virtio-mem driver started up.

The virtio-mem driver already supports the "kdump mode", where it won't
hotplug any memory but instead queries the device to implement the
pfn_is_ram() callback, to avoid reading unplugged memory holes when reading
the vmcore.

With this series, if the virtio-mem driver is included in the kdump
initrd -- which dracut already takes care of under Fedora/RHEL -- it will
now detect the device RAM ranges on s390 once it probes the devices, to add
them to the vmcore using the same callback mechanism we already have for
pfn_is_ram().

To add these device RAM ranges to the vmcore ("patch the vmcore"), we will
add new PT_LOAD entries that describe these memory ranges, and update
all offsets vmcore size so it is all consistent.

My testing when creating+analyzing crash dumps with hotplugged virtio-mem
memory (incl. holes) did not reveal any surprises.

Patch #1 -- #7 are vmcore preparations and cleanups
Patch #8 adds the infrastructure for drivers to report device RAM
Patch #9 + #10 are virtio-mem preparations
Patch #11 implements virtio-mem support to report device RAM
Patch #12 activates it for s390, implementing a new function to fill
          PT_LOAD entry for device RAM

Who is merging this?
virtio parts:

Acked-by: Michael S. Tsirkin <mst@redhat.com>

I guess this series should go via Andrew Morton. Andrew?

Acked-by: Heiko Carstens <hca@linux.ibm.com> # s390