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Fri, 25 Oct 2019 08:02:30 +0000 (UTC) Subject: Re: [PATCH 0/7] i386: Add `machine` parameter to query-cpu-definitions To: Christian Borntraeger , Eduardo Habkost , qemu-devel@nongnu.org References: <20191025022553.25298-1-ehabkost@redhat.com> <6e7d171e-18c4-6835-f89c-e9e66c093d62@de.ibm.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <4cd530f9-54f3-80e7-1b66-c91f71160062@redhat.com> Date: Fri, 25 Oct 2019 10:02:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <6e7d171e-18c4-6835-f89c-e9e66c093d62@de.ibm.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-MC-Unique: q1qmSX4LM6aOXticw-XDBQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252; format=flowed Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Thomas Huth , Janosch Frank , Cornelia Huck , Markus Armbruster , Paolo Bonzini , Igor Mammedov , Jiri Denemark , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 25.10.19 09:55, Christian Borntraeger wrote: >=20 >=20 > 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 r= eally 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 fe= atures 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 w= e decided to always model what you would see under LPAR and are able to spe= cify for a KVM guest. So you can use "z13" in an up-to-date LPAR environmen= t, 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 u= ses query-cpu-model-expansion() to convert this model+machine to a machine-= independent representation. That representation is sufficient for all use c= ases we were aware of (esp. "virsh domcapabilities", the host CPU model, mi= gration). >> >> While s390x has versioned CPU models, we have no explicit way of specify= ing them for older machines, besides going over query-cpu-model-expansion a= nd using expanded "base model + features". >> >> I can see that this might make sense on x86-64, where you only have mayb= e 3 versions of a CPU (e.g., the one Intel messed up first - Haswell, the o= ne 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 wan= t 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 f= eatures", but at least for expanding the host model on s390x, it is not rea= lly helpful. When migrating, the model expansion does the trick. >> >> I haven't looked into details of "how to specify or model versions". May= be 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. >=20 > 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-onLPA= R >=20 > Instead we do check if we can construct an equivalent model on the migrat= ion 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 fe= nce of > facilities for old machine versions. >=20 > For example > -machine s390-virtio-ccw-3.1 -cpu z14 will not have the multiple epoch fa= cility > and > -machine s390-virtio-ccw-4.0 -cpu z14 will have the multiple epoch facili= ty. > As migration does always require the tuple of machine and cpu this is sav= e. I fail > to see what the benefit of an explicit z14-3.1 would be. >=20 AFAIKS the only real benefit of versioned CPU models is that you can add=20 new CPU model versions without new QEMU version. Then you can specify "-cpu z13-vX" or "-cpu z13 -cpuv X" (no idea how=20 versioned CPU model were implemented) on any QEMU machine. Which is the=20 same as telling your customer "please use z13,featX=3Don" in case you have= =20 a good reason to not use the host model (along with baselining) but use=20 an explicit model. If you can change the default model of QEMU machines, you can automate=20 this process. I am pretty sure this is a corner case, though (e.g.,=20 IBRS). Usually you have a new QEMU machine and can simply enable the new=20 feature from that point on. --=20 Thanks, David / dhildenb