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

From: Heiko Carstens <hca@linux.ibm.com>
To: David Hildenbrand <david@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	qemu-devel@nongnu.org, Halil Pasic <pasic@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.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: Wed, 15 Jul 2020 18:14:51 +0200	[thread overview]
Message-ID: <20200715161451.GB3934@osiris> (raw)
In-Reply-To: <30ba06e7-5e84-9683-5b37-623f40b3a6db@redhat.com>

On Wed, Jul 15, 2020 at 01:42:02PM +0200, David Hildenbrand wrote:
> > So, are you saying that even at IPL time there might already be memory
> > devices attached to the system? And the kernel should _not_ treat them
> > as normal memory?
> 
> Sorry if that was unclear. Yes, we can have such devices (including
> memory areas) on a cold boot/reboot/kexec. In addition, they might pop
> up at runtime (e.g., hotplugging a virtio-mem device). The device is in
> charge of exposing that area and deciding what to do with it.
> 
> The kernel should never treat them as normal memory (IOW, system RAM).
> Not during a cold boot, not during a reboot. The device driver is
> responsible for deciding how to use that memory (e.g., add it as system
> RAM), and which parts of that memory are actually valid to be used (even
> if a tprot might succeed it might not be valid to use just yet - I guess
> somewhat similar to doing a tport on a dcss area - AFAIK, you also don't
> want to use it like normal memory).
> 
> E.g., on x86-64, memory exposed via virtio-mem or virtio-pmem is never
> exposed via the e820 map. The only trace that there might be *something*
> now/in the future is indicated via ACPI SRAT tables. This takes
> currently care of indicating the maximum possible PFN.

Ok, but all of this needa to be documented somewhere. This raises a
couple of questions to me:

What happens on

- IPL Clear with this special memory? Will it be detached/away afterwards?
- IPL Normal? "Obviously" it must stay otherwise kdump would never see
  that memory.

And when you write it's up to the device driver what to with that
memory: is there any documentation available what all of this is good
for? I would assume _most likely_ this extra memory is going to be
added to ZONE_MOVABLE _somehow_ so that it can be taken away also. But
since it is not normal memory, like you say, I'm wondering how that is
supposed to work.

As far as I can tell there would be a lot of inconsistencies in
userspace interfaces which provide memory / zone information. Or I'm
not getting the point of all of this at all.

So please provide more information, or a pointer to documentation.