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: <ZxEf6NOs1hDFZd1E@tuxmaker.boeblingen.de.ibm.com>
Date: Thu, 17 Oct 2024 16:32:08 +0200
From: Alexander Gordeev <agordeev@...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,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...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 Thu, Oct 17, 2024 at 02:07:12PM +0200, David Hildenbrand wrote:
> On 17.10.24 12:00, David Hildenbrand wrote:
> > Well, DIAGNOSE 260 is z/VM only and DIAG 500 is KVM only. So there are
> > currently not really any other reasonable ways besides SCLP.
> 
> Correction: Staring at the code again, in detect_physmem_online_ranges()
> we will indeed try:
> 
> a) sclp_early_read_storage_info()
> b) diag260()

So why care to call diag260() in case of DIAGNOSE 500? What about the below?

void detect_physmem_online_ranges(unsigned long max_physmem_end)
{
	if (!sclp_early_read_storage_info()) {
		physmem_info.info_source = MEM_DETECT_SCLP_STOR_INFO;
	} else if (physmem_info.info_source == MEM_DETECT_DIAG500_STOR_LIMIT) {
		unsigned long online_end;

		if (!sclp_early_get_memsize(&online_end)) {
			physmem_info.info_source = MEM_DETECT_SCLP_READ_INFO;
			add_physmem_online_range(0, online_end);
		}
	} else if (!diag260()) {
		physmem_info.info_source = MEM_DETECT_DIAG260;
	} else if (max_physmem_end) {
		add_physmem_online_range(0, max_physmem_end);
	}
}

> But if neither works, we cannot blindly add all that memory, something is
> messed up. So we'll fallback to
> 
> c) sclp_early_get_memsize()
> 
> But if none of that works, something is seriously wrong.

Ok, thanks for the clarification.