linux-kernel - Re: [PATCH v2 4/7] s390/physmem_info: query diag500(STORAGE LIMIT) to support QEMU/KVM memory devices

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Message-ID: <20241015152008.7641-P-hca@linux.ibm.com>
Date: Tue, 15 Oct 2024 17:20:08 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Eric Farman <farman@...ux.ibm.com>
Cc: David Hildenbrand <david@...hat.com>, 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>,
        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 Tue, Oct 15, 2024 at 11:01:44AM -0400, Eric Farman wrote:
> On Mon, 2024-10-14 at 20:43 +0200, Heiko Carstens wrote:
> > On Mon, Oct 14, 2024 at 04:46:16PM +0200, David Hildenbrand wrote:
...
> > +#define DIAG500_SC_STOR_LIMIT 4
...
> I like the idea of a defined constant here instead of hardcoded, but maybe it should be placed
> somewhere in include/uapi so that QEMU can pick it up with update-linux-headers.sh and be in sync
> with the kernel, instead of just an equivalent definition in [1] ?
> 
> [1] https://lore.kernel.org/qemu-devel/20241008105455.2302628-8-david@redhat.com/

It is already a mess; we have already subcode 3 defined:

#define KVM_S390_VIRTIO_CCW_NOTIFY 3

in

arch/s390/include/uapi/asm/virtio-ccw.h

which for some reason is uapi. But it doesn't make sense to put the
new subcode 4 there too. So what is the end result?

Another uapi file? I think resolving this would be a project on its own.