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

From: Christian Borntraeger <borntraeger@de.ibm.com>
To: David Hildenbrand <david@redhat.com>, Heiko Carstens <hca@linux.ibm.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>,
	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: Mon, 13 Jul 2020 13:08:19 +0200	[thread overview]
Message-ID: <92b1a2a6-2348-d4ff-6d20-35f3bfef710a@de.ibm.com> (raw)
In-Reply-To: <07E9FD5B-F07F-415B-9C00-A2A882F07CBC@redhat.com>

On 13.07.20 12:27, David Hildenbrand wrote:
> 
> 
>> 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?> 

Wouldnt sclp be the right thing to provide the max increment number? (and thus the max memory address)
And then (when I got the discussion right) use diag 260 to get the _current_ value.