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

From: David Hildenbrand
Subject: Re: [PATCH RFC 2/5] s390x: implement diag260
Date: Thu, 9 Jul 2020 20:15:49 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

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 :)


David / dhildenb

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