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Message-ID: <59199850-f9cd-4249-92b5-40904facc11b@redhat.com>
Date: Mon, 14 Oct 2024 21:42:34 +0200
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 PĂ©rez
<eperezma@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
Jonathan Corbet <corbet@....net>, Mario Casquero <mcasquer@...hat.com>
Subject: Re: [PATCH v2 4/7] s390/physmem_info: query diag500(STORAGE LIMIT) to
support QEMU/KVM memory devices
On 14.10.24 20:43, Heiko Carstens wrote:
> On Mon, Oct 14, 2024 at 04:46:16PM +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).
>>
>> Tested-by: Mario Casquero <mcasquer@...hat.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>> arch/s390/boot/physmem_info.c | 46 ++++++++++++++++++++++++++--
>> arch/s390/include/asm/physmem_info.h | 3 ++
>> 2 files changed, 46 insertions(+), 3 deletions(-)
>
> ...
>
>> +static int diag500_storage_limit(unsigned long *max_physmem_end)
>> +{
>> + register unsigned long __nr asm("1") = 0x4;
>> + register unsigned long __storage_limit asm("2") = 0;
>> + unsigned long reg1, reg2;
>> + psw_t old;
>
> In general we do not allow register asm usage anymore in s390 code,
> except for a very few defined places. This is due to all the problems
> that we've seen with code instrumentation and register corruption.
Makes sense. Note that I was inspired by GENERATE_KVM_HYPERCALL_FUNC
that also still uses register asm usage.
>
> The patch below changes your code accordingly, but it is
> untested. Please verify that your code still works.
Below LGTM, I'll give it a churn, thanks!
--
Cheers,
David / dhildenb
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