Re: [PATCH RFC 2/5] s390x: implement diag260

From: Christian Borntraeger <borntraeger@de.ibm.com>
To: David Hildenbrand <david@redhat.com>, qemu-devel@nongnu.org
Cc: Thomas Huth <thuth@redhat.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Halil Pasic <pasic@linux.ibm.com>,
	qemu-s390x@nongnu.org, Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH RFC 2/5] s390x: implement diag260
Date: Thu, 9 Jul 2020 12:52:56 +0200	[thread overview]
Message-ID: <efe7e845-2e0f-96ba-7e29-c6ac74db9e2f@de.ibm.com> (raw)
In-Reply-To: <20200708185135.46694-3-david@redhat.com>

On 08.07.20 20:51, David Hildenbrand wrote:
> Let's implement the "storage configuration" part of diag260. This diag
> is found under z/VM, to indicate usable chunks of memory tot he guest OS.
> As I don't have access to documentation, I have no clue what the actual
> error cases are, and which other stuff we could eventually query using this
> interface. Somebody with access to documentation should fix this. This
> implementation seems to work with Linux guests just fine.
> 
> The Linux kernel supports diag260 to query the available memory since
> v4.20. Older kernels / kvm-unit-tests will later fail to run in such a VM
> (with maxmem being defined and bigger than the memory size, e.g., "-m
>  2G,maxmem=4G"), just as if support for SCLP storage information is not
> implemented. They will fail to detect the actual initial memory size.
> 
> This interface allows us to expose the maximum ramsize via sclp
> and the initial ramsize via diag260 - without having to mess with the
> memory increment size and having to align the initial memory size to it.
> 
> This is a preparation for memory device support. We'll unlock the
> implementation with a new QEMU machine that supports memory devices.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

I have not looked into this, so this is purely a question. 

Is there a way to hotplug virtio-mem memory beyond the initial size of 
the memory as specified by the  initial sclp)? then we could avoid doing
this platform specfic diag260?
the only issue I see is when we need to go beyond 4TB due to the page table
upgrade in the kernel. 

FWIW diag 260 is publicly documented. 