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

From: David Hildenbrand <david@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.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 20:15:49 +0200	[thread overview]
Message-ID: <c86e5b12-883a-8522-a15c-9af3804e003c@redhat.com> (raw)
In-Reply-To: <efe7e845-2e0f-96ba-7e29-c6ac74db9e2f@de.ibm.com>

On 09.07.20 12:52, Christian Borntraeger wrote:
> 
> 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?

We need a way to tell the guest about the maximum possible PFN, so it
can prepare for that. E.g. on x86-64 this is usually done via ACPI SRAT
tables. On s390x, the only way I see is using a combination of diag260,
without introducing any other new mechanisms.

Currently Linux selects 3. vs 4 level page tables based on that size (I
think that's what you were referring to with the 4TB limit). I can see
that kasan also does some magic based on the value ("populate kasan
shadow for untracked memory"), but did not look into the details. I
*think* kasan will never be able to track that memory, but am not
completely sure.

I'd like to avoid something as you propose (that's why I searched and
discovered diag260 after all :) ), especially to not silently break in
the future, when other assumptions based on that value are introduced.

E.g., on my z/VM LinuxOne Community Cloud machine, diag260 gets used as
default, so it does not seem to be a corner case mechanism nowadays.

> 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. 

Yeah, Conny pointed me at the doc - makes things easier :)

-- 
Thanks,

David / dhildenb