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

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Message-ID: <20241030092324.6264-E-hca@linux.ibm.com>
Date: Wed, 30 Oct 2024 10:23:24 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: David Hildenbrand <david@...hat.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 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.
But anyway, let's move on.

Acked-by: Heiko Carstens <hca@...ux.ibm.com>

However, I would like to see an Ack or review from Alexander Gordeev
or Vasily Gorbik for this patch.