Re: [PATCH 0/7] i386: Add `machine` parameter to query-cpu-definitions

From: David Hildenbrand <david@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>,
	Eduardo Habkost <ehabkost@redhat.com>,
	qemu-devel@nongnu.org
Cc: Thomas Huth <thuth@redhat.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Markus Armbruster <armbru@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Jiri Denemark <jdenemar@redhat.com>,
	Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH 0/7] i386: Add `machine` parameter to query-cpu-definitions
Date: Fri, 25 Oct 2019 10:02:29 +0200	[thread overview]
Message-ID: <4cd530f9-54f3-80e7-1b66-c91f71160062@redhat.com> (raw)
In-Reply-To: <6e7d171e-18c4-6835-f89c-e9e66c093d62@de.ibm.com>

On 25.10.19 09:55, Christian Borntraeger wrote:
> 
> 
> On 25.10.19 09:17, David Hildenbrand wrote:
>> On 25.10.19 04:25, Eduardo Habkost wrote:
>>> We had introduced versioned CPU models in QEMU 4.1, including a
>>> method for querying for CPU model versions using
>>
>> Interesting, I was not aware of that.
>>
>> On s390x, we somewhat have versioned CPU models, but we decided against giving them explicit names (e.g., z13-v1 or z13-4.1.0), because it didn't really seem to be necessary. (and we often implement/add features for older CPU models, there is a lot of fluctuation) Actually, you would have had to add "z13-z/VM-x.x.x" or "z13-LPAR-x.x.x" or "z13-KVM-x.x.x" to model the features you actually see in all the different virtual environments ("what a CPU looks like"). Not to talk about QEMU versions in addition to that. So we decided to always model what you would see under LPAR and are able to specify for a KVM guest. So you can use "z13" in an up-to-date LPAR environment, but not in a z/VM environment (you would have to disable features).
>>
>> Each (!base) CPU model has a specific feature set per machine. Libvirt uses query-cpu-model-expansion() to convert this model+machine to a machine-independent representation. That representation is sufficient for all use cases we were aware of (esp. "virsh domcapabilities", the host CPU model, migration).
>>
>> While s390x has versioned CPU models, we have no explicit way of specifying them for older machines, besides going over query-cpu-model-expansion and using expanded "base model + features".
>>
>> I can see that this might make sense on x86-64, where you only have maybe 3 versions of a CPU (e.g., the one Intel messed up first - Haswell, the one Intel messed up next - Haswell-noTSX, and the one that Intel eventually did right - Haswell-noTSX-IBRS) and you might want to specify "Haswell" vs. "Haswell-IBRS" vs. "Haswell-noTSX-IBRS". But actually, you will always want to go for "Haswell-noTSX-IBRS", because you can expect to run in updated environments if I am not wrong, everything else are corner cases.
>>
>> Of course, versioned CPU model are neat to avoid "base model + list of features", but at least for expanding the host model on s390x, it is not really helpful. When migrating, the model expansion does the trick.
>>
>> I haven't looked into details of "how to specify or model versions". Maybe IBM wants to look into creating versions for all the old models we had. But again, not sure if that is of any help for s390x. CCing IBM.
> 
> I agree that this does not look very helpful.
> Especially as several things depend on the kernel version a QEMU version is
> not sufficient to be guarantee construction success.
> So we would need something like z14-qemu4.0-kernel-5.2-suse-flavour-onLPAR
> 
> Instead we do check if we can construct an equivalent model on the migration target.
> And that model is precise. We do even have versions.
> Right now with QEMU on s390  our models are versioned in a way that we fence of
> facilities for old machine versions.
> 
> For example
> -machine s390-virtio-ccw-3.1 -cpu z14 will not have the multiple epoch facility
> and
> -machine s390-virtio-ccw-4.0 -cpu z14 will have the multiple epoch facility.
> As migration does always require the tuple of machine and cpu this is save. I fail
> to see what the benefit of an explicit z14-3.1 would be.
> 

AFAIKS the only real benefit of versioned CPU models is that you can add 
new CPU model versions without new QEMU version.

Then you can specify "-cpu z13-vX" or "-cpu z13 -cpuv X" (no idea how 
versioned CPU model were implemented) on any QEMU machine. Which is the 
same as telling your customer "please use z13,featX=on" in case you have 
a good reason to not use the host model (along with baselining) but use 
an explicit model.

If you can change the default model of QEMU machines, you can automate 
this process. I am pretty sure this is a corner case, though (e.g., 
IBRS). Usually you have a new QEMU machine and can simply enable the new 
feature from that point on.

-- 

Thanks,

David / dhildenb