Message-ID: <35fc960b-0013-4264-93d6-6511d54ab474@redhat.com>
Date: Wed, 30 Oct 2024 10:42:05 +0100
From: David Hildenbrand <david@...hat.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-s390@...r.kernel.org, virtualization@...ts.linux.dev,
 linux-doc@...r.kernel.org, kvm@...r.kernel.org,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Thomas Huth <thuth@...hat.com>,
 Cornelia Huck <cohuck@...hat.com>, Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Eric Farman <farman@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>,
 Mario Casquero <mcasquer@...hat.com>
Subject: Re: [PATCH v3 3/7] s390/physmem_info: query diag500(STORAGE LIMIT) to
 support QEMU/KVM memory devices

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@...hat.com>
>> Signed-off-by: David Hildenbrand <david@...hat.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

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