Hi! On Thu, Sep 22, 2016 at 02:47:36PM -0400, Jason J. Herne wrote:
Hi Jiri & Eduardo,
You might remember a discussion with David Hildenbrand of IBM on the Qemu mailing list regarding a new Qemu<->Libvirt interface for cpu modeling. I am picking up this work from David and I wanted to confirm that we are still on the same page as to the direction of that interface.
I am, but I would like to hear Jiri's feedback in case there's anything that is still lacking in the new interface. Additionanl comments below:
For your reference: https://www.redhat.com/archives/libvir-list/2016-June/thread.html#01413 https://lists.gnu.org/archive/html/qemu-devel/2016-09/threads.html#00489
The first link is to the discussion you were directly involved in. The second link is to the final patch set posted to qemu-devel. The cover letter gives a good overview of the interface added to Qemu and the proposed use-case for Libvirt to use this new cpu modeling support. I'll paste in the most relevant section for your convenience:
--------------------------------Libvirt usecase---------------------------- Testing for runability: - Simply try to start QEMU with KVM, compat machine, CPU model
The main problem here is for libvirt to get machine-friendly error information from QEMU in case the VM fails to start. But if libvirt avoids starting QEMU based on previously-collected runnability information, we should be OK.
- Could be done using query-cpu-model-comparison in the future.
I sent a series adding runnability information to query-cpu-definitions as well.
Identifying host model, e.g. "virsh capabilities" - query-cpu-model-expansion on "host" with "-M none --enable-kvm"
Correct.
<cpu mode='host-model'>: - simply copy the identified host model
<cpu mode='host-passthrough'>: - "-cpu host"
Correct.
"virsh cpu-baseline": - query-cpu-model-baseline on two models with "-M none"
"virsh cpu-compare": - query-cpu-model-comparison on two models with "-M none"
Correct.
There might be some cenarios where libvirt wants to convert another CPU model to a static variant, this can be done using query-cpu-model-expansion ---------------------------------------------------------------------------
Now that I've hopefully refreshed your memory :) I just want to make sure that you are still on-board with this plan. I realize that this new approach does things differently than Libvirt does today for other platforms. Especially x86_64. The big differences are as follows:
1. We will invoke qemu to gather the host cpu data used for virsh capabilities. Today this data seems to be collected directly from the host hardware for most (all?) architectures.
I believe libvirt developers want to change this, because the current approach is wrong for x86 as well.
2. virsh cpu-models {arch} will also use a Qemu invocation to gather cpu model data.
I believe libvirt can collect this data once, and cache it (like it does today).
3. Most architectures seem to use a "map" (xml file containing cpu model data that ships with Libvirt) to satisfy #1 and #2 above. Our new method does not use this map as it gets all of the data directly from Qemu.
I believe the goal is to make cpu_map.xml unnecessary on x86 as well.
4. virsh cpu-baseline and cpu-compare will now invoke qemu directly as well.
Note: I'm not sure exactly how much all of this will apply just to s390 with other architectures moving over to the new interface if/when they want to. Or if we will want to change all architectures to this new interface at the same time. Any guidance?
I am not sure about this part, because I don't know what are the existing use cases of cpu-baseline and cpu-compare. But I believe this shouldn't be a problem as long as the function is not called very often and/or results are cached by libvirt. -- Eduardo