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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: Mon, 13 Jul 2020 12:27:01 +0200

> Am 13.07.2020 um 11:12 schrieb Heiko Carstens <hca@linux.ibm.com>:
> On Fri, Jul 10, 2020 at 05:24:07PM +0200, David Hildenbrand wrote:
>>> On 10.07.20 17:18, Heiko Carstens wrote:
>>> On Fri, Jul 10, 2020 at 02:12:33PM +0200, David Hildenbrand wrote:
>>>>> Note: Reading about diag260 subcode 0xc, we could modify Linux to query
>>>>> the maximum possible pfn via diag260 0xc. Then, we maybe could avoid
>>>>> indicating maxram size via SCLP, and keep diag260-unaware OSs keep
>>>>> working as before. Thoughts?
>>>> Implemented it, seems to work fine.
>>> The returned value would not include standby/reserved memory within
>>> z/VM. So this seems not to work.
>> Which value exactly are you referencing? diag 0xc returns two values.
>> One of them seems to do exactly what we need.
>> See
>> https://github.com/davidhildenbrand/linux/commit/a235f9fb20df7c04ae89bc0d134332d1a01842c7
>> for my current Linux approach.
>>> Also: why do you want to change this
>> Which change exactly do you mean?
>> If we limit the value returned via SCLP to initial memory, we cannot
>> break any guest (e.g., Linux pre 4.2, kvm-unit-tests). diag260 is then
>> purely optional.
> Ok, now I see the context. Christian added my just to cc on this
> specific patch.

I tried to Cc you an all patches but the mail bounced with unknown address 
(maybe I messed up).

> So if I understand you correctly, then you want to use diag 260 in
> order to figure out how much memory is _potentially_ available for a
> guest?

Yes, exactly.

> This does not fit to the current semantics, since diag 260 returns the
> address of the highest *currently* accessible address. That is: it
> does explicitly *not* include standby memory or anything else that
> might potentially be there.

The confusing part is that it talks about „adressible“ and not „accessible“. 
Now that I understood the „DEFINE STORAGE ...“ example, it makes sense that the 
values change with reserved/standby memory.

I agree that reusing that interface might not be what we want. I just seemed 
too easy to avoid creating something new :)

> So you would need a different interface to tell the guest about your
> new hotplug memory interface. If sclp does not work, then maybe a new
> diagnose(?).

Yes, I think a new Diagnose makes sense. I‘ll have a look next week to figure 
out which codes/subcodes we could use. @Christian @Conny any ideas/pointers?

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