Re: [PATCH v3 3/7] s390/physmem_info: query diag500(STORAGE LIMIT) to support QEMU/KVM memory devices

David Hildenbrand
2024-10-30T09:42:12Z

On 30.10.24 10:23, Heiko Carstens wrote:
> On Fri, Oct 25, 2024 at 04:14:48PM +0200, David Hildenbrand wrote:
>> To support memory devices under QEMU/KVM, such as virtio-mem,
>> we have to prepare our kernel virtual address space accordingly and
>> have to know the highest possible physical memory address we might see
>> later: the storage limit. The good old SCLP interface is not suitable for
>> this use case.
>>
>> In particular, memory owned by memory devices has no relationship to
>> storage increments, it is always detected using the device driver, and
>> unaware OSes (no driver) must never try making use of that memory.
>> Consequently this memory is located outside of the "maximum storage
>> increment"-indicated memory range.
>>
>> Let's use our new diag500 STORAGE_LIMIT subcode to query this storage
>> limit that can exceed the "maximum storage increment", and use the
>> existing interfaces (i.e., SCLP) to obtain information about the initial
>> memory that is not owned+managed by memory devices.
>>
>> If a hypervisor does not support such memory devices, the address exposed
>> through diag500 STORAGE_LIMIT will correspond to the maximum storage
>> increment exposed through SCLP.
>>
>> To teach kdump on s390 to include memory owned by memory devices, there
>> will be ways to query the relevant memory ranges from the device via a
>> driver running in special kdump mode (like virtio-mem already implements
>> to filter /proc/vmcore access so we don't end up reading from unplugged
>> device blocks).
>>
>> Update setup_ident_map_size(), to clarify that there can be more than
>> just online and standby memory.
>>
>> Tested-by: Mario Casquero <mcasquer@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> arch/s390/boot/physmem_info.c | 47 +++++++++++++++++++++++++++-
>> arch/s390/boot/startup.c | 7 +++--
>> arch/s390/include/asm/physmem_info.h | 3 ++
>> 3 files changed, 54 insertions(+), 3 deletions(-)
>
> Looks like I couldn't convince you to implement a query subcode.

Well, you convinced me that it might be useful, but after waiting on
feedback from the KVM folks ... which didn't happen I moved on. In the
cover letter I have "No query function for diag500 for now."

My thinking was that if we go for a query subcode, maybe we'd start
"anew" with a new diag and use "0=query" like all similar instructions I
am aware of. And that is then a bigger rework ...

... and I am not particularly interested in extra work without a clear
statement from KVM people what (a) if that work is required and; (b)
what it should look like.

Thanks for the review Heiko!

--
Cheers,

David / dhildenb